First we need to know that as bulbs burn out, the voltage in the Christmas tree lights increases in the remaining bulbs and that can cause them to burn out.
The problem may be potentially a bad fuse. Most string lights have two tiny fuses inside the plug.
Unplug the lights of the pre-lit Christmas tree then look at the plug that goes into the wall : open the fuse compartment it and you’ll see two small fuses, check it’s probably one of them is blown.
Replace it with a new one (remember where you put the two extra fuses when you bought your strand of lights..).
If you follow correctly the steps on this video, you potentially can fix Christmas tree lights not working issue.
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