When repairing Christmas lights, it's important to first identify the issue that needs to be fixed. Is the strand of lights not working at all, or is it just a few bulbs that are out? Once the problem is identified, you can start by checking the fuses. If the fuse is blown, replace it. If that doesn't work, try testing each bulb individually to see if any are damaged or burned out. If the bulbs are okay, check the wiring for any breaks or loose connections. Finally, if the lights still don't work, you may need to replace the entire strand.