I like these little solar LED light strings, they're cheap and they actually work decently in my experience. Except when moisture gets in, and you get some corrosion and they don't work.
It's worth checking the battery terminal. Sometimes stuff builds up there, and you can just use a wire brush to get it off. Sometimes it's something much worse, like a problem in the string, but a lot of times it's something like this and you have to take it apart a couple times a year.







