You can set it back to OS X from within OS 9. Go for it.

Alternately, you might want to try booting into single-user mode and running fsck on that filesystem. Reboot the computer, and when you hear the initial chime, hold down command-s until you see something happening.

You'll be given a command-line interface, and, IIRC, you'll get automatically logged in as root. Run ``/sbin/fsck -y''. After it's done, run ``/sbin/shutdown -r now''.