Either your database is full, or your disk is full.

I saw a similar problem in slightly different context last week.  IIRC, the disk on which the database resides was close to full, and I think that the increment by which the database gets auto extended was larger than the available free space.  I truncated the SiteLog and EventLog tables, cleaned up the disk, and that solved the problem.  You could also do a backup of the database to also take care of the transaction logs, if you've configured that to happen on backup.

You can also do that I ultimately did to keep this from happening, at least for a longer while:  I moved my database to a disk that had LOTS of free space.

I also seem to recall that Mitchel Sellers has a blog entry about keeping your database clean and healthy (mitchelsellers.com).