Some people have accidentally written data onto the portion of their
harddisk that contains the disk driver code used by the Mac OS.  If
this happens, and the partition map still indicates there is a valid
driver present, then the computer may well lock-up whenever it tries
to read from that drive in the boot sequence.  The solution is to re-
install the driver.  That may be easier said than done because it may
seem impossible to boot the machine. If you can talk to open firmware
to request a floppy boot, then the "help*.coff utility" available at
ftp://ftp.linuxppc.org/users/harry/ will allow you to use pdisk to delete
the driver partition.  You should then be able to boot.  If you can't
get to open firmware, then another approach is to unplug the power
connector from the dead drive, boot the computer (e.g. from CD), then
with the computer still running, plug the power connector back into the
drive, then run drive setup or whatever driver/formatting utility you
like to install a new driver.
 
        harry eaton  haceaton@aplcomm.jhuapl.edu
 [wj@duke.edu - fixed location of the help utility to Harry's LinuxPPC directory]

It could also be as simple as corrupted PRAM - try zapping it during bootup
by holding down the [command]-[option]-p-r key combination while the computer
boots (during and after the startup chord - let it reset twice, at least).
 
wj@acpub.duke.edu

Some people have noted that command-option-P-R may not work if you have
extra devices connected to your ADB bus.  If it doesn't work, disconnect
any such devices and try again.

Paul.Mackerras@cs.anu.edu.au

        The easy workarounds were the Sys8 Install Disk (then use HD Setup from the utility folder to mount the ATA volumes and copy the files off) or any SCSI disk seems to work also (both Syquest units booted -- they had system files on them -- from the Question Mark screen).
        In either event, you'll probably have to reformat the ATA disk and reload all your software. No other fix I tried (including blanking out and rebuilding the driver partitions) worked.
        If someone comes up with a more elegant fix, please supersede this.