Just as a thought, can you log in via the command line? If so then have look in in /tmp see if there are any large files, use the commands
du -hs (Directory Usage) - to see how much space given folders take up (-hs makes it an easy to read format, ie. in Gig/Meg rather than bytes)
and use df /dev/hda (or hdb depending on your drive) to see exactly how much room there is on the drive.
If you can't log at all - then perhaps grab a live distro and boot up in that to check disk usage.
As to what may have chewed it up - not sure, you're not running MythTV are you - I had a similar problem of leaving MythTV running and finding out it was trying to record some programs I'd forgotten to cancel and using up masses of drive space.
Hope this helps (if only a little)