over heating problem

I left my pc on for the night, in the morning, it shut down by itself. I re-powered it on, and after a few minutes it shut down by itself. I rebooted abain, same thing. after a few times, during the bios, a voice said "temperature too high" or something like that. I went into the bios, and my cpu is like 57 celcius, going up. I have an AMD 64 3200.
It's weird, cause my pc didn't do that before... what's the problem?
my mobo is an asus k8vse deluxe

edit: it gone up to 60 celcius. I checked the fan, it's turning...

 

I would suggest reapplying thermal grease before doing anything else. Use the good stuff man, that "Cheapobrand!!" won't do you any good.

 

the problem was too much dust between the heat sink and the fan. removed the dust, works like a charm.