Before you put them away neatly coiled last year you checked them and they worked.
You took them out this year and they have inextricably entangled themselves; and they don’t work.
You wonder why, but you needn’t. The answer is in the name, Fairy Lights. It is, of course, the fairies who make them work.
But where ever there are fairies there are hobgoblins and it is their work to sabotage them.
When you pack them away this year my advice is to leave a small piece of goose cheese in with them. Hobgoblins hate the stuff.