I agree, while it sounds like you have a very stable system there must be a reason why the power cut out all of a sudden. It might have been to do with the power socket itself (do you have a surge protector? This may have cut the power to protect your PC if there was a sudden power spike for some reason) or it might be something inside your PC. Being 2.5 years old, taking off the cover probably isn't going to harm any warranty (but do check first.)
No need for anything drastic, just look for anything that might be amiss... muck clogging up the fans, power cable become a bit loose somewhere, etc.
If it does happen more regularly it could also indicate that your PSU is on its way out, although unless it's a really cheap make I wouldn't have thought that'd be an issue. It's possible though. I'd say don't worry but keep an eye on it.