Access your bios on boot and insure that it is reading your harddrive(s). If not, the usual cause is a loose or bad ribbon cable. Open up the case and insure that both the drive connectors and the mobo connectors are in solid. If drives are all showing, check boot order to insure it's not seeking on the wrong drive.
If the harddrives are showing up in your bios, you prolly have a corrupted MBR (master boot record). You did make an Emergency boot disk, dintcha? Insert EBD and rebbot computer...when you get the A:\> type in " sys c:" that should transfer the system files and start the boot process. If not, reboot to A:\> type in "fdisk/mbr" that will repair your master boot record. Remove emergecy floppy and reboot. All should be well. If not, you may have virus damage, or hard drive failure.