Christmas lights
There are a lot of reasons why Christmas lights may stop working, but you only need to know the basics. There could be a problem with the fuses that are usually found at the ends of the strand of lights. The fuses have the purpose to prevent overloading, melting and the starting of a fire.
Some strands of lights stop working when only one bulb burns out or when one bulb is missing. In each of these cases use the replacement bulbs given with the lights. Also if a portion of the voltage burns out then the strand will stop functioning to prevent overloading.
The problem may not be with the lights themselves but with the power supply. See if the electrical outlet you are using to power the lights is working. Notice that if some of the lights are working the problem cannot be with the power source.