What is probably happening

What to do

Dead CMOS battery, CMOS instructions lost; if running SCSI, the adapter lost its marbles

When you boot up, go into the CMOS menu. You typically do this by holding down the delete key after you turn power on. Inside most CMOS menus, you'll find an autodetect function. This will reprogram the chipset to recognize your hard drive(s) and other devices. Ditto for SCSI adapters.