I'm guessing it no longer sees a hard drive? CMOS battery dead on an old motherboard?
If that is the problem you need to figure out how to interrupt the boot sequence (what key combo to enter as it starts to boot).
Once in CMOS you need to reset the hard drive settings if they are lost, but if they 'see' the hard drive already don't touch them .
Anyway I'd search the net for the info on setting up a hard drive in cmos/bios on your brand of motherboard/computer. If it is so old that bios lacks autodetect, you will also need the info about the hard drive (C/H/S/ (cylinder head sectors), etc, but that is going back awhile. If it is all original you should be ok.
All that said this might not be the issue, but it was a common problem in the 90's.
**** Caution, I haven't worked on an old rig in decades, and my A+ cert expired over 20 years ago .