To answer your question, you have to answer some questions too. What? Well, let's start with the basic, what OS and version does your phone have? What was the last thing you were doing before it "auto-rebooted"? Did you root your AF2?
If you do not prefer to answer the questions for additional details and just prefer to get an answer then ANSWER: do a FACTORY RESET and see if that will do the same thing you encountered. Chances are it won't unless there's a hardware issue BUT that doesn't stop it from happening again even after that process. This is why we get to the bottom of it by asking those questions to pin point the possible "exact" reason why you encountered it.