zap P ram

I updated my 2007 build Imac to OS10.7.5 in preparation for mountain Lion.
I backed up to time machine.
I repaired permissions and zapped the ram.
on restart my computer cannot find the hard drive (folder with ? mark)
I retarted with snow leopard install disc, tried to run disc utility but disc utility cannot find the hard drive.
any suggestions?
thanks

Update to Zap P Ram
I installed new system on an external firewire drive.
(snow leopard, big download updates, installed Lion, even bigger download updates)
Days later:
Lion Disk utility mounted the drive and repaired it so that I could restart with the "LOST" drive.
Still not 100% though.
Installed tech tools Pro 6, and am making a Clone disc image saved to my Mac Pro.
Interestingly restarted with the Lost drive Lion Recovery( with CMD+R ) on the same "Lost" drive and successfully navigated to my time machine backup.
Once I back up and make cloned disc image, I will try to repair disk, failing that I will reformat and Recover from either of my 2 options.
Just thought I would let you know the progress
Regards
David

You should test your hard drive. It sounds like it may be failing. See my tips for how to do so more thoroughly.
http://www.mac-forums.com/forums/os...o-much-space-used-hard-drive.html#post1372971