It's difficult to know without more specifics. What was the phone doing when it overheated? What apps were running? How did you know it overheated? What happens when you plug it in (does a power/charging light come on)?
Beyond that, it's quite possible that the battery is bad. A battery fault could lead both to overheating, and also to the battery's protective circuitry shutting it off it to prevent further damage/overheating. Unfortunately, I don't personally know anything about the process for replacing that particular battery, and it's not made to be user-replaceable. That being said, you might want to look at this link: [post|305213]
but of course this voids the warranty.