Our master bathroom was out a couple days ago. It might have been obvious to be that something had been chained off the GFCI that the hot tub was on, but we quickly established that wasn't it. In the process of tracking it I tried tripping the GFCI by the sink, before finding the panel had one tripped breaker, but then the bathroom still didn't work... turned out both the GFCI by the sink and the upstream breaker had gone.