The easiest way to troubleshoot a light bulb is to change it. If that doesn’t work, it probably doesn’t have power. Here are some things to check when there is no power at the light(s). For switched lights, see Troubleshoot Switches

A voltage detector can be placed near the light fixture line wire, a lamp cord, or inside a bulb socket to indicate if voltage is present.

How to Troubleshoot LED Tubes

Some LED tubes may be connected to a ballast, others are directly connected to line voltage. If they are connected directly to line voltage, check that voltage is present. If they are connected to a ballast, see below.

How to Troubleshoot Fluorescent and LED Tubes and Ballasts

Fluorescent tube lights can be difficult to troubleshoot because they have a separate ballast. When fluorescent and LED tubes don’t light up, it could be a bad tube or ballast. If there are known good tubes of the same diameter or type in another fixture, install them in the fixture that does not light up. If it still doesn’t light up, then most likely the ballast is bad, or there is no power to the fixture. Less likely, there could be a loose wire connection in the fixture. If a fluorescent tube is darkened at one end, it is likely that the tube is bad. See picture below.

Troubleshoot Fluorescent Tubes with a Rapid Start Ballast

If a fixture has a two lamp rapid start ballast, and one tube is bad, the other tube will not light up. It is preferred to replace both tubes when one goes bad anyway.

Bad Fluorescent Tube with a Burned End

How to Troubleshoot Light Fixtures

If a light fixture has a switch, it could have failed. Sometimes the tab inside the socket could be bent down or burned. If it is burned, you will be able to see the damage. If you think the tab is bent, turn off power to the socket. Using a needle nose pliers, pull out the tab a little bit.

How to Troubleshoot Low Voltage Lighting

Low voltage lighting uses a transformer to convert line voltage to low voltage (12V or 24V).

Test for line voltage at the line side of the driver/transformer. A voltage detector or multimeter can be used.
Test for low voltage at the load side of the driver/transformer using a multimeter (a voltage detector does not measure low voltage).
If some lights work, test for low voltage at each light.

How to Troubleshoot Metal Halide and Sodium Lights

Test for line voltage at the line side of the ballast.
Replace the lamp.
Replace the ballast.

Always use a voltage detector on a known good power source to verify the detector is working properly. Find Voltage detectors