So, what happens if a strand of lights goes out after you’ve decorated the tree? It’s usually one bad bulb that causes a whole string to go defunct. Check each bulb on the strand to make sure it is properly seated. Tightening a loose one may do the trick. If it doesn’t, use an extra bulb that came with the set, and swap it into each bulb socket until you find the bum light. Once you do, all the lights should come back on.