MacOS X needs at least 10% (maybe 2 GB?) of free space to work, so if you maxed out your HD, that might be the problem.
Can you boot from the system software install CD? If so, and if you've got everything backed up, then reformat the drive and reinstall.
Otherwise: You should get DiskWarrior and try to boot from the CD to repair the HD. Check how much empty HD space you have!
Finally: Put a larger HD in. They're cheap nowadays, and you can put your original HD (once you've resurrected it) into an external firewire case.