Plug in the strand to an electrical outlet to make sure all the lights are working. Many strands come with replacement bulbs, if you keep them in a place where you can find them. The older incandescent light strands are more likely to all stop working if one or two lights burn out. LED lights, on the other hand, do not tend to have the same problem and also last longer and are more energy efficient.
Remember: Do not mix LEDs with incandescent strands on the same circuit.
