Jump into single user mode and see if there's anything there. Reboot the system, and when there's a prompt to pass arguments, type "1" and hit enter (that's how it works on RHEL and OpenSuSE, at least). If the normal partitions aren't mounted, you can mount them manually, and see what's going on. I would guess, however, that this is just a symptom of a failing hard drive.
Otherwise, give us more information about the system:
- How long since it was last rebooted
- Any changes made since last reboot
- Boot loader changes (i.e. switching from LILO to GRUB or vice-versa)
- Why you needed to reboot