All Lights Are Out
Check the transformer for power. Look at the circuit breaker to make sure it has not tripped. Make sure the transformer is still plugged into the outlet.
Inspect all of the cable between the lights and the transformer. Look for bare wires that might have shorted out the system, and make sure none of the wires have cuts in them.
Check the wire connections inside the individual lights. Sometimes these come loose. Locate the terminal connections (you may need to refer to the instruction manual) and reattach or tighten the wires where necessary.
One Light Is Dim or Out
If the bulb has a broken or burned-out filament, replace it.
If it does not look burned out, replace the bulb with a known good bulb. If it still does not light, the socket is bad. Replace the fixture.
Inspect the fixture housing for water or moisture buildup. Make sure the watertight fittings are not wet and are snugly connected.