What's the recent history of the computer?
Is that a Pentium II computer running Widows 98?

Motherboard batteries can go bad and cause the system BIOS to revert to default settings. If this happens, the hard drive may not be listed as a boot device or be detected at all.

Replace the motherboard battery, make sure the hard drive is detected in system BIOS, and then set the hard drive as the primary boot device/volume.