Well, from the little information I have, I would theorise that it could be that you have overloaded the CPU, how old is your PC, what OS is it running, what spec is your PC and what were you doing at the time? You might have accidently overloaded it. This might have been becasue you upgraded something on your computer but forgot about making sure that your current heatsink/fan was capable of dealing with it. What I am saying next I doubt very much, seeing as you have come to this forum without a clue as to why it has happened, were you overclocking your PC? I already know your answer, but you do sometimes get people who are doing something like this, think they have come up with a solution for every eventuallity, and then something happens, and they are "possitive" that what they were doing was not the problem becasue they had counter measures in place. Why not open up your Case and take a poke around inside, don't touch anything unless you know what you are doing, just see if nothing looks burnt or looks strange and out of place. I do hope I am wrong and that it is something else that has caused it, something relatively cheap.