I've had this happen to me twice. It's a right fuckin pisser!
When you boot up try the option "boot with last known working configuration" or whatever it is - I can't remember exactly. The first time it happened, this worked for me after a few attempts, although it did take about 20mins to boot.
The second time this didn't work. Get into your bios and disable the option of auto rebooting when the bootup fails. This means you'll get a blue screen with an error message. At least then you'll know what it is, and might have a chance of sorting it out.
This was the one I was getting:
"The registry cannot load the hive (file):
\SystemRoot\System32\Config\SOFTWARE
or its log or alternate.
It is corrupt, absent, or not writeable."