Posted 07 August 2007 - 08:09 AM
What OS are you using? And by BIOS, you mean you can't even see the BIOS screen that comes up after your PC posts? (beeps?) If you can, do you mean that the keys you use to enter the BIOS settings don't respond at all? Also, when your computer completely froze, did you get a bluescreen? Or did the PC simply lock up and stop responding? Or did it fully restart?
Need a little more info, but from what you've said, I'd say your hard drive either had an error on it, provoked by the defrag, or defragmenter caused an unrecoverable error. This happened to me recently when using Vista and installing a driver with fast RAM timings. Sometimes it just happens. Usually small hard drive errors can be recovered, but sometimes (if you're attorciously unlucky) they can destroy your OS just like that.