Why did my automatic winding watch start working again?

I have a fossil self winding watch and one day i was messing around with it and it stopped working? No matter how much i shook the watch to make to pendulum spin, the watch still wouldn't move. About a month later I picked the watch up and shook it a bit to get the pendulum spinning and it started back up again. What could be the cause of this so i can prevent it from happening again?