Sounds a lot like what my pc did.

My powersupply was faulty and it stopped working(slowly stopped) without any indication anything was really wrong. PC kept freezing and restarting. Took it in under warrenty, replaced power supply and now it works just like new.

* The thing just didn't shut off or anything, it rebooted to windows, shut down and such after time. I couldn't run anything. GL though.

~ KAMAHAME---Oh shit it's happening again.... ~