First try booting in Safe mode by holding down the Shift key. If that works go to Applications - Utilities - Disk Utility and verify your disk. Repair if necessary. If this doesn't work:

Boot from your installation disk in the CD drive, Wait till the second of third page till the pull down menu appears then go to Utilities - Disk Utility and verify your disk. Repair if necessary.

If this fails reinstall the system. Still can't see the drive? ome back and we'll tell you how to replace your drive.