Nothing is more aggravating than carefully decorating your tree only to discover that the lights won't work. While Christmas tree lights come in many shapes and colors, there are some basic guidelines to follow so your tree lights work properly.
Plug the lights into an electrical outlet.
Turn the light switch and the circuit breaker on.
Check the fuse or fuses from the plug at the end of the light strand (usually the glass-cylinder type with a wire filament). Use a small screwdriver to pop out the tab and open the plug. If the fuse is charred or burned out, replace it with a new fuse, and plug the light strand in your outlet.
If the fuse isn't the problem, there's a problem with a bulb. Untangle all your string lights, and check for loose or frayed wires, damaged sockets or broken bulbs. Discard any damaged string lights.
Replace any damaged bulbs with spare ones. Make sure to use bulbs with the same voltage rating as the rest of your string lights.
If no bulbs are damaged, test each bulb with a bulb tester (an inexpensive plastic mold or pen-shaped tool with a 9-volt battery inside). Follow the manufacturer’s instructions to test each bulb. Or use an extension cord to test each light strand.
Once you find the faulty bulb, unplug the light strand from the outlet, and replace it with the new bulb. Firmly seat each bulb in its socket.