Christmas lights are wired to stay lit, even if one of the bulbs burns out; but, this doesn't always happen. Sometimes a couple of bulbs or the entire light string doesn't light up when you plug the cord into a power outlet. When this happens, use a non-contact voltage detector to determine exactly which light bulb is faulty.
Plug the lights into an outlet. Sit on the floor near the outlet, so that you can easily access the lights from the outlet to the end of the string.
Turn on the non-contact voltage detector. Place the non-contact voltage detector near the light bulb. You do not have to touch the light bulb for the voltage detector to work. If the voltage detector lights up, an electrical current is present at that location and the bulb is good. If the voltage detector does not light up, that indicates a bad bulb.
Replace the bad bulb, and then try the lights. If the lights do not work test all of the bulbs and replace any bad ones. If this does not solve the problem, there may be a fault in the wiring of the light string.
Discard the string of lights if they are not working because of faulty wiring.