Want to be a holiday hero? My wife gave me a bag of broken light sets, and I set to work learning how to diagnose and repair string lights. tldr: buy one of these tools that can quickly restore function and isolate failed bulbs. www.amazon.com/dp/B000R8… (referral link)
A non-obvious (to me) fact about light sets: the voltage (120V in the US) is divided among the working bulbs in a series circuit. So if you continue operating with burned-out bulbs, it puts increasing strain on the ones that are still working. Always be replacing!