Woke up to a dead watch

I keep my instinct fully charge during the week and am wondering what happened to my watch, I woke up the other morning and it was off, I never turned it off so that was strange. When I turned it on, it had 1 bar and so it appears something drained the battery? There was no activity tracking happening, and the watch was charged the day before and had full charge. Any ideas of how I can find out what happened?