My GFCI test button won’t push in when I try to reset it, but nothing has tripped in the breaker box, and the hairdryer works when I plug it into outlets down the hall. Why won’t my GFCI reset?

The outdoor outlets and bathroom lights didn't work. I knew enough to check for tripped circuit breakers and GFCI outlets. But I couldn't find the problem. I was just about to start pulling apart ...

See? Good and evil. An RCD (or GFCI in the States) is a kind of circuit breaker that trips when the amount of current in the hot and neutral mains power lines aren’t equal and opposite ...