Your landscape lighting worked perfectly last night.

Tonight, the first six lights still turn on—but everything after them is completely dark.

Or maybe an entire section of your backyard suddenly stopped working after new plants were installed.

The transformer is on.

Other lighting zones still work.

So where is the problem?

One possibility is a break, damaged section, or failed connection in the buried landscape-lighting wire.

The frustrating part is that the cable may be hidden beneath:

• Mulch
• Grass
• Soil
• Gravel
• Planting beds
• Pavers
• Landscaping

Fortunately, you usually don't need to dig up the entire yard.

The key is to troubleshoot systematically.

Instead of searching randomly, determine:

Where does good voltage stop?

Once you identify the last point where power is present and the first point where it disappears, you've dramatically reduced the area that needs inspection.

In this guide, we'll explain how to find a break in buried low-voltage landscape-lighting wire using:

• Visual inspection
• Voltage testing
• Divide-and-test troubleshooting
• Known fixture locations
• Connection inspection
• Wire-tracing equipment
• Continuity testing when appropriate

Important: This guide focuses on the low-voltage side of a landscape-lighting system. Do not open or test 120V transformer input wiring unless you are qualified to work on line-voltage electrical equipment.

What Causes Buried Landscape Lighting Wire to Break?

Direct-burial landscape cable is designed for outdoor use, but that doesn't make it impossible to damage.

Common causes include:

Shovels and Gardening Tools

This is probably one of the most common causes.

Someone plants:

• A tree
• Shrub
• Flower
• Irrigation line
• Fence post

and accidentally cuts the buried lighting cable.

Because landscape cable is often routed through planting beds, it can be directly in the path of future gardening work.

Lawn Edgers

Cable installed near walkway or lawn borders can be damaged by edging equipment.

Repeated contact may eventually cut through the insulation or conductor.

Landscape Renovation

New projects can disturb old wiring.

Common examples include:

• New retaining walls
• Artificial turf
• Irrigation repairs
• Drainage work
• Patio construction
• Pool installation
• Tree removal
• New fencing

If the lights stopped working immediately after construction or landscaping, start your search there.

Corroded Underground Connections

Sometimes the cable itself isn't broken.

The problem is a splice.

Outdoor connections are exposed to:

• Soil moisture
• Irrigation
• Rain
• Fertilizer
• Temperature changes

If moisture reaches the copper conductors, corrosion can increase electrical resistance or eventually create a completely open circuit.

Kings specifically warns that poorly protected outdoor splices can corrode and cause early failure on its direct-burial wire product information.

Animal Damage

Rodents and other animals may damage exposed or shallow cable.

This is more common where wire:

• Emerges from the ground
• Runs behind shrubs
• Passes through crawl areas
• Remains exposed beside structures

Tree Roots and Soil Movement

Roots may shift, stretch, or compress old cable.

While good landscape wire is flexible, years of landscaping changes can place stress on connections and conductor jackets.

Previous Repairs

A lighting system that has been modified several times may contain:

• Old buried splices
• Unknown cable routes
• Mixed wire gauges
• Abandoned branches
• Poorly protected repairs

Sometimes the most likely failure point is a repair made years earlier.

First Determine Whether You Really Have a Broken Wire

A dark lighting zone doesn't automatically mean the cable has been cut.

Other possible causes include:

• Transformer turned off
• Timer schedule
• Photocell
• Smart-zone control
• Failed bulb
• Failed fixture
• Loose transformer terminal
• Corroded connector
• Excessive voltage drop
• Failed transformer output
• Overload protection

Before digging, verify that the problem is actually in the cable path.

Step 1: Check the Transformer

Start at the beginning of the system.

Confirm:

• Transformer is powered
• Manual mode works
• Timer isn't shutting the system off
• Photocell isn't preventing operation
• Correct zone is enabled
• Low-voltage branch wire is connected securely

If another branch connected to the same transformer works normally, that's useful information.

It suggests that the transformer itself may still be operating.

Kings currently offers both AC and DC transformer types, so make sure you know which power type your system uses before testing voltage.

Step 2: Test Transformer Output With a Multimeter

Use a digital multimeter on the low-voltage output side.

For an AC landscape transformer:

Set meter to AC volts — V~

For a DC power supply:

Set meter to DC volts — V⎓

Measure across the appropriate low-voltage output terminals.

If the transformer provides normal output, continue downstream.

If it provides zero output even though it should be operating, investigate the transformer/control system before assuming the buried wire is broken.

Step 3: Identify the Last Working Fixture

This is one of the most useful troubleshooting clues.

Imagine ten landscape lights are connected along one cable run.

Lights 1–6:

Working.

Lights 7–10:

Dead.

The break is unlikely to be between:

Transformer and Light 1

because Lights 1–6 still have power.

Instead, focus on the area between:

Light 6 and Light 7

That immediately reduces your search area.

Why the Last Working Light Matters

Landscape-lighting cable frequently follows the same physical route as the fixtures.

A common daisy-chain layout might look like:

Transformer → Light 1 → Light 2 → Light 3 → Light 4 → Light 5

If Light 3 works and Lights 4 and 5 do not, likely trouble spots include:

• Light 3 connection
• Cable leaving Light 3
• Cable between Lights 3 and 4
• Light 4 connection

You probably don't need to excavate the cable between the transformer and Light 1.

Step 4: Measure Voltage at the Last Working Fixture

Use the multimeter to test the wire around the last fixture that still operates.

Suppose you measure:

12.1V

That confirms power is reaching that point.

Now move to the next fixture.

Step 5: Test the First Dead Fixture

Measure voltage at the first fixture that does not operate.

Scenario A

Last working fixture:

12.1V

First dead fixture:

0V

This strongly suggests a break or failed connection between those locations.

Scenario B

Last working fixture:

12.1V

First dead fixture:

11.9V

Power is reaching the dead fixture.

The problem is more likely:

• Bulb
• Fixture
• Socket
• Local fixture connection

rather than the buried main cable.

The Most Important Troubleshooting Rule

Use this simple principle:

Last good voltage → first bad voltage = search area

If you follow that rule, a 300-foot lighting system can often be reduced to a 10- or 20-foot section that requires inspection.

Step 6: Inspect Connections Before Digging Up Cable

Before you start trenching, inspect the connections at both ends of the suspected section.

A failed splice is often easier to fix than buried cable damage.

Look for:

• Loose connector
• Green or blackened copper
• Moisture
• Broken conductor
• Wire pulled out of connector
• Cracked connector body
• Improperly stripped cable

Kings offers IP67 landscape-lighting connectors designed for outdoor wiring from 12–18 AWG.

For repair work where a more permanent splice is desired, Kings also carries brass screw-tight crimp connectors with heat-shrink weather protection.

A Connector Can Look Fine and Still Be Bad

Don't rely only on appearance.

A splice can look completely normal on the outside while the conductor inside is:

• Corroded
• Loose
• Partially broken

If voltage is good before the connection but disappears immediately afterward, remake the splice using an appropriate outdoor-rated connection method.

Step 7: Look for Recent Digging

Before using specialized locating equipment, think about what happened around the property recently.

Ask:

• Was a tree planted?
• Was irrigation repaired?
• Did someone install a fence?
• Was new sod installed?
• Did landscapers replace shrubs?
• Was a drainage pipe installed?
• Did anyone use a shovel near the light run?

A cable failure that appears immediately after landscape work is rarely a coincidence.

Look for Visual Clues

Even buried wire often leaves clues.

Inspect for:

• Fresh soil
• New planting holes
• Trenches
• Irrigation repairs
• Stakes
• Fence posts
• Utility work
• Damaged mulch
• Exposed wire

Start your search around those areas.

Step 8: Estimate the Cable Route

If you didn't install the original system, try to reconstruct where the wire probably travels.

Landscape cable commonly follows:

• Fixture rows
• Walkways
• Planting beds
• Property lines
• Retaining walls

It often takes the most direct practical route between fixtures.

But don't assume the previous installer followed the route you would have chosen.

Older systems can contain unexpected turns and abandoned wiring.

How Deep Is Landscape Lighting Wire Usually Buried?

Installation depth varies by project, local requirements, and cable-routing conditions.

Low-voltage landscape cable is often installed relatively shallow compared with many utility lines.

That makes it easier to install—but also easier to damage with landscaping tools.

Always follow applicable codes and manufacturer requirements for your specific installation.

Before digging, also make sure you aren't confusing landscape cable with other underground utilities.

Use appropriate utility-location procedures before excavation.

Step 9: Use Divide-and-Test Troubleshooting

If the suspected section is still long, divide it.

Suppose you know the break lies somewhere between:

Light 4

and

Light 10

Instead of digging from Light 4 all the way to Light 10, test somewhere near the middle.

For example:

Light 7

If Light 7 has good voltage, the fault is farther downstream.

If Light 7 has zero voltage, the fault lies between Light 4 and Light 7.

You just cut the search area roughly in half.

Repeat the process.

Example: Finding a Break Quickly

Imagine a 12-fixture path-light system.

Lights:

1–5 work

6–12 are dead

Measurements:

Transformer:

12.5V

Light 5:

12.0V

Light 6:

0V

You inspect the connections at Lights 5 and 6.

They appear intact.

There is approximately 18 feet of buried cable between them.

You notice that a new shrub was recently planted halfway between those two fixtures.

Carefully exposing the cable near the planting hole reveals a shovel cut.

The entire 12-light system was narrowed down to an 18-foot section before any serious digging began.

What If All Fixtures on One Branch Are Dead?

Suppose the transformer has several separate wire runs.

Branch A:

Working.

Branch B:

Working.

Branch C:

Completely dead.

Test Branch C at the transformer terminal.

If voltage is present at the transformer but absent at the first fixture, the fault lies somewhere between those points.

Possible causes include:

• Loose transformer connection
• Failed buried splice
• Broken main cable
• Hub/junction failure

What If Every Landscape Light Is Dead?

If the entire system goes dark simultaneously, a buried cable break is less likely than a problem common to the entire system.

Check:

• Transformer
• Timer
• Photocell
• GFCI
• Smart control
• Main low-voltage connection

One underground cut usually affects one branch or everything downstream of the damaged point—not every independent circuit connected to the transformer.

What If One Fixture Is Dead but the Next Fixture Works?

That is an important clue.

If:

Light 4 is off

but:

Light 5 is on

then the main cable must still be carrying power beyond Light 4.

The problem is likely local to:

• Light 4 fixture
• Bulb
• Fixture lead
• Connector

rather than a complete break in the main landscape cable.

Voltage Drop vs. Broken Wire

A broken wire and excessive voltage drop can look different.

Broken Wire

Typical symptom:

Full voltage → suddenly 0V

Example:

Light 4 = 12.1V

Light 5 = 0V

Light 6 = 0V

Voltage Drop

Typical symptom:

Voltage gradually decreases.

Example:

Transformer = 12.5V

Light 1 = 12.2V

Light 4 = 11.4V

Light 8 = 10.2V

That pattern suggests:

• Long cable
• Excessive load
• Undersized wire
• High-resistance connections

rather than a complete conductor break.

Kings' 12/2 direct-burial cable uses 100% copper conductors and is specifically positioned for long-distance low-voltage landscape-lighting applications where voltage-drop reduction matters.

What Is a Landscape Lighting Wire Tracer?

When voltage testing narrows the fault but you still can't find the cable, a wire tracer or underground cable locator can help.

These tools generally consist of:

• Signal transmitter
• Handheld receiver

The transmitter applies a traceable signal to the cable.

The receiver detects that signal through the ground.

This can help identify:

• Cable route
• Approximate location of buried wire
• Areas where the signal changes or disappears

Wire tracers are particularly useful when:

• You didn't install the original system.
• Cable paths are unknown.
• The run travels through a large planting bed.
• There are no fixtures near the damaged area.

Wire Tracer vs. Multimeter

These tools solve different problems.

Multimeter

Answers:

Is voltage present here?

Best for:

• Transformer testing
• Fixture testing
• Voltage drop
• Narrowing the break location

Wire Tracer

Answers:

Where does this buried cable physically go?

Best for:

• Unknown cable routes
• Locating buried runs
• Reducing unnecessary digging

For difficult repairs, the two tools work extremely well together.

Can a Wire Tracer Find the Exact Break?

Sometimes it can help narrow the break location significantly, but results depend on:

• Tracer type
• Soil conditions
• Cable depth
• Other buried wires
• Moisture
• Installation layout

Don't assume every inexpensive tracer will identify a buried conductor break to the exact inch.

Think of it as another tool for shrinking the search zone.

How to Use a Wire Tracer Safely

Follow the instructions for the specific tracer.

Some tracing methods require the landscape cable to be:

De-energized and disconnected

before connecting the transmitter.

Do not connect a wire tracer to an energized circuit unless the manufacturer's instructions specifically permit that operating method.

Never connect low-voltage tracing equipment to the 120V line side of the transformer unless the tool and operator are specifically qualified for that work.

Can You Use Continuity to Find a Broken Wire?

Yes, continuity testing can confirm whether a conductor is electrically complete.

But:

The cable must be completely de-energized before continuity or resistance testing.

Never switch the multimeter to continuity or ohms while the landscape circuit is powered.

The wire should also be properly isolated from equipment that could affect the measurement.

Voltage Testing Is Often Easier

For many residential troubleshooting situations, voltage testing is enough.

If the system is energized and you determine:

12V here

and:

0V twenty feet later

you already know the fault lies somewhere between those points.

You may not need a continuity test at all.

How to Repair a Cut Landscape Lighting Wire

Once you've located the damage:

1. Turn the transformer off.
2. Confirm the low-voltage circuit is de-energized.
3. Expose enough undamaged cable on both sides.
4. Remove damaged conductor and insulation.
5. Use a connector suitable for outdoor/direct-burial landscape wiring.
6. Seal the repair according to the connector manufacturer's instructions.
7. Restore power.
8. Measure voltage after the repair.
9. Test all downstream fixtures.
10. Rebury and protect the repair appropriately.

Do not simply twist the wires together and cover them with ordinary indoor electrical tape.

Why Waterproof Connections Matter

Underground repairs are exposed to moisture.

If water reaches bare copper, corrosion can gradually increase resistance.

The lighting may initially work.

Then months later you may see:

• Flickering
• Dimming
• Intermittent operation
• Complete failure

Kings recommends waterproofing landscape-wire splices and currently offers multiple connection products for wet outdoor environments.

Should You Replace the Entire Cable or Repair It?

It depends on the condition of the run.

Repair Is Often Appropriate When:

• Damage is localized.
• The rest of the wire is in good condition.
• Cable gauge is appropriate.
• There are only one or two failures.

Replacement May Be Better When:

• Cable has many old repairs.
• Insulation is badly deteriorated.
• Wire gauge is undersized.
• Voltage drop is excessive.
• Route is poorly designed.
• Multiple sections have been damaged.

Sometimes repairing a twenty-year-old cable for the fifth time makes less sense than installing one clean new run.

Choosing Replacement Landscape Wire

If replacement is necessary, use cable designed for low-voltage outdoor installation.

Kings currently offers multiple direct-burial copper wire sizes, including:

• 14/2
• 12/2
• 10/2
• 8/2

The current 12/2 product uses 100% copper conductors, is direct-burial rated, and is available in 100-, 250-, and 500-foot rolls.

The 14/2 product is also direct-burial rated and uses 100% copper conductors.

For particularly demanding long runs, heavier cable such as 8/2 is also available.

Don't Automatically Replace With the Same Gauge

If the original system suffered severe voltage drop, replacing damaged 16/2 cable with new 16/2 cable may preserve the same electrical problem.

Before replacing a long section, evaluate:

• Fixture wattage
• Run length
• Transformer voltage
• Existing wire gauge
• Future expansion

A larger conductor may make sense.

Map the Wire Before Reburying It

Once you've repaired or replaced the cable, create a simple record.

You can:

• Take photos
• Draw a property sketch
• Measure distance from walls or walkways
• Record fixture order
• Note splice locations

This can save enormous time during future landscaping and troubleshooting.

Take Photos Before Covering the Trench

Photograph:

• Cable path
• Junction locations
• Connections
• Nearby fixed landmarks

Years later, a photo showing that the cable runs 18 inches from the retaining wall can be far more useful than guessing.

Mark Major Junctions

Where practical, keep important splice or hub locations accessible.

A serviceable junction is much easier to troubleshoot than a connection buried somewhere beneath several inches of soil with no record of its location.

Prevent Future Wire Breaks

Finding the current fault is useful.

Preventing the next one is better.

Avoid Planting Directly Over Cable Routes

Leave reasonable separation when possible.

Document the Wiring

Create a simple landscape-lighting map.

Protect Cable in High-Risk Areas

Use appropriate protective methods where wire crosses areas likely to be disturbed.

Tell Landscapers Where Wiring Runs

Especially before:

• Tree planting
• Irrigation work
• Fence installation

Inspect Exposed Cable

Don't leave wire where lawn equipment can hit it.

Common Mistakes When Looking for a Buried Wire Break

Mistake #1: Digging Before Testing

Use a multimeter first.

Mistake #2: Assuming the Transformer Is Bad

If other branches work, the transformer may be fine.

Mistake #3: Replacing Every Dead Fixture

Multiple downstream lights rarely fail simultaneously because every bulb went bad at once.

Mistake #4: Ignoring the Last Working Light

It provides one of your most important clues.

Mistake #5: Ignoring Connections

Splices fail more often than many people expect.

Mistake #6: Using Continuity Mode on a Live Circuit

Never do this.

Mistake #7: Repairing With Indoor Connectors

Use connections suitable for outdoor landscape wiring.

Mistake #8: Burying the Repair Without Testing

Restore power and verify downstream voltage first.

Mistake #9: Forgetting to Document the Repair

Take photos while the wire is exposed.

Troubleshooting Decision Tree

Use this simplified process.

Entire system dead?

Check:

Transformer → controls → power source

One branch dead?

Check:

Transformer branch terminal → first fixture

Several lights work, then everything goes dark?

Check:

Last working fixture → first dead fixture

Only one fixture dead, but fixtures after it work?

Check:

Local fixture → bulb → connector

Lights become gradually dimmer?

Check:

Voltage drop → wire gauge → run length → load

Voltage disappears suddenly?

Check:

Break → splice → cut conductor

Example: New Fence Installation

A homeowner has twelve path lights along the property line.

After a fence contractor installs several posts:

Lights 1–7 work.

Lights 8–12 are dead.

Voltage:

Light 7:

12.0V

Light 8:

0V

One new fence post is located directly between those fixtures.

That should immediately become the first inspection location.

Example: Irrigation Repair

A backyard lighting branch works intermittently after a sprinkler repair.

Voltage before one buried connector:

12.2V

Voltage after connector:

Sometimes 12V, sometimes 4V, sometimes 0V

That pattern points strongly toward a loose, wet, or corroded connection rather than a transformer problem.

Example: Long Run With No Actual Break

Ten lights gradually get dimmer.

Measurements:

Transformer:

12.5V

Light 1:

12.1V

Light 5:

10.9V

Light 10:

9.4V

This isn't the classic signature of a cut wire.

Instead, investigate:

• Wire gauge
• Total wattage
• Cable length
• Connections
• Transformer tap
• Wiring layout

Don't dig up the cable looking for a nonexistent complete break.

When Should You Use a New Wire Run Instead?

Sometimes finding the exact break is more work than installing a new cable.

A replacement run may make more sense when:

• Existing path is inaccessible.
• Wire runs beneath permanent structures.
• Multiple old failures exist.
• Cable gauge is inadequate.
• A better route is available.
• Future expansion is planned.

A clean new run can eliminate several old buried splices at once.

Shop Landscape-Lighting Wire and Repair Components

If troubleshooting reveals damaged cable or failed connections, Kings Outdoor Lighting carries the core components needed for low-voltage landscape-lighting repairs.

12/2 Direct-Burial Copper Landscape Wire
100% copper, direct-burial-rated low-voltage cable available in multiple roll lengths.
Shop 12/2 landscape wire

14/2 Direct-Burial Copper Landscape Wire
A 14-gauge, two-conductor option for compatible low-voltage outdoor installations.
Shop 14/2 landscape wire

Speed Connect Landscape Lighting Connector
An IP67 connector designed for landscape-lighting wire from 12–18 AWG.
Shop Speed Connect wire connectors

Brass Crimp + Heat-Shrink Connectors
Weather-resistant repair connectors designed for low-voltage landscape-lighting wire.
Shop brass crimp connectors

Low-Voltage Landscape Lighting Transformers
Compare AC, DC, multi-tap, timer, and smart transformer options.
Shop landscape-lighting transformers

Final Verdict: Don't Dig Until You Know Where Power Stops

The fastest way to find a buried landscape-lighting wire break is usually not to start digging.

Start testing.

Follow the electricity.

Transformer → First fixture → Last working fixture → First dead fixture

Once you identify:

Good voltage here

and:

No voltage there

you have isolated the section containing the fault.

Then:

1. Inspect connections.
2. Look for recent digging.
3. Trace the suspected cable route.
4. Use a wire tracer if necessary.
5. Carefully expose only the affected section.
6. Repair using outdoor-rated components.
7. Test before reburying.

A systematic approach can turn what looks like a massive underground wiring problem into one small repair.

And once the cable is exposed, document the route.

Future you—or the next installer—will be very glad you did.

Frequently Asked Questions

How do you find a break in buried landscape-lighting wire?

Start by identifying the last fixture with good voltage and the first fixture with no voltage. The break or failed connection is usually somewhere between those points.

How do I know if my landscape-lighting wire is broken?

Common signs include several downstream lights suddenly going dark while upstream fixtures continue working, or measuring normal voltage at one point and zero volts farther down the cable.

Can a multimeter find a broken landscape wire?

A multimeter can help isolate the section containing the break by showing where voltage disappears. It doesn't physically show where the buried cable is located.

Can a wire tracer locate landscape-lighting wire?

Yes. An appropriate underground wire tracer can help follow the physical path of buried low-voltage cable.

What is the difference between a multimeter and a wire tracer?

A multimeter measures electrical conditions such as voltage. A wire tracer helps identify where a cable physically travels underground.

What causes buried landscape-lighting wire to fail?

Common causes include shovel damage, landscaping, fence posts, irrigation work, corrosion, poor splices, lawn equipment, animals, and old repairs.

Why do some of my landscape lights work while the rest are dead?

If the fixtures are connected along the same cable run, the fault may be located between the final working fixture and the first dead fixture.

What if one light is dead but the next one still works?

The main cable is likely still intact. Inspect the dead fixture, bulb, fixture lead, and local connector.

Can a bad wire connector make several lights stop working?

Yes. A failed connection on a daisy-chain or branch can disconnect every fixture located downstream.

How do I find a bad landscape-lighting splice?

Measure voltage before and after the connection. If normal voltage is present before the splice but disappears or drops significantly afterward, inspect or remake that connection.

How can I tell voltage drop from a broken wire?

Voltage drop usually produces a gradual reduction in voltage along the run. A complete break typically causes voltage to suddenly fall to zero beyond the damaged location.

Should I use continuity mode to find a broken wire?

Continuity testing can help, but the circuit must be completely de-energized and appropriately isolated before resistance or continuity testing.

Can I continuity-test landscape wire while the transformer is on?

No. Never use resistance or continuity mode on an energized circuit.

How deep is low-voltage landscape-lighting wire buried?

Depth varies by installation and applicable requirements. Always follow local regulations, manufacturer guidance, and safe excavation practices.

Can a shovel cut landscape-lighting wire?

Yes. Buried low-voltage lighting cable can be damaged by shovels, fence posts, irrigation work, and other landscaping activities.

Can tree roots damage landscape-lighting wire?

Roots and soil movement can place stress on old wiring and connections, although direct mechanical damage from landscaping activities is often easier to identify.

How do I repair cut landscape-lighting wire?

Turn the transformer off, expose clean undamaged cable, remove the damaged section, and reconnect using an outdoor-rated waterproof or direct-burial connection method suitable for the cable.

Can I repair landscape wire with electrical tape?

Ordinary indoor electrical tape alone is not an appropriate permanent underground splice method. Use a connector designed for the outdoor or direct-burial environment.

Should landscape-lighting connections be waterproof?

Yes. Outdoor landscape splices should use connection methods suited to wet and damp environments to help protect the copper conductors from moisture and corrosion.

Should I replace a damaged wire or repair it?

A localized cut can often be repaired. Replacing the run may be better when the wire has multiple old failures, inadequate gauge, deteriorated insulation, or excessive voltage drop.

What wire should I use to replace damaged landscape cable?

Use appropriately sized outdoor-rated direct-burial low-voltage cable. Wire gauge should be selected according to load, voltage, cable length, and allowable voltage drop.

Is 12/2 wire good for landscape lighting?

12/2 copper cable is commonly used for low-voltage landscape-lighting installations and can be especially useful for longer or more heavily loaded runs, but the correct size depends on the actual project.

Why did my landscape lights stop working after landscaping?

Recent digging may have cut the cable or disturbed an existing underground connection. Start your inspection near the area where work occurred.

Should I dig up the entire landscape-lighting wire run?

Usually not. Voltage testing can often narrow the problem to a small section between the last point with good voltage and the first point without power.

When should I call a professional?

Call a qualified professional if the problem involves the 120V transformer input, GFCI or breaker issues, overheating, repeated electrical faults, complex buried wiring, or any situation you're not comfortable troubleshooting safely.