Unplug the Christmas lights from the outlet
Locate the fuse compartment on the plug
Slide open the fuse cover
Remove the old fuse with a small tool or your fingers
Check the fuse type and rating on the old fuse or light string label
Insert a replacement fuse with the same type and rating
Replace the fuse cover
Plug the lights back in to test them
If the lights still do not work, check for other damaged bulbs or wiring