Since your computer completely froze, it's likely your Motherboard or one of your expansion cards (i.e. video cards). Basically, there's a sudden change to the way your hardware is behaving that the operating system can't cope. My bet is on the motherboard (from prior experience in similiar issues). One thing you can do is to make sure all the little power cables (the little square 4-pin ones) are solidly connected. You may want to go a bit further and literally disconnect everything and reconnect it back together to eliminate the possibility of a loose cable or connector. Have a can of compressed air handy. (Watch out for static electricity and water too. I killed a computer by the air can freezing and dripping water onto the motherboard once. Oops.)
In general, intermittent problems like yours are the hardest things to troubleshoot, so don't get too bent out of shape over any difficulty figuring this one out.
In general, intermittent problems like yours are the hardest things to troubleshoot, so don't get too bent out of shape over any difficulty figuring this one out.