So much is missing in your post but here's a common issue with 1+ year old machines. Watch and do what Brian documents in this fine Cnet production:
I thought the big blue screen was a thing of the past but now I am seeing it again. It only happens when I am watching a video on the Internet (via Firefox). It happened three times yesterday. Somebody thinks it could be my video card - any other ideas? Oh and by the way the machine booted itself twice yesterday (not doing video at the time!