He immediately knew what to do. You have to unwind the two strings of lights. The one with the screw in lights may be 20 years old but still works. You just have to change burned bulbs. The other string is newer and more disposable.
The plugs work in one way of connecting and not the other.
The strings are a bit of a long bundle to twirl around the tree and also to plug to the lit star at the top. The star has never given any problems. Dad knows this stuff!