It's called sudden cardiac death, and can happen to anyone who is predisposed to it. How do you know if you're predisposed to it? You don't. Basically the heart rhythms get all fucked up and causes the heart to stop. If you happen to have a defibrillator handy, usually no problem, otherwise, c'ya! Usually happens to one or two high school athletes a year in most large cities and makes the news.