Can’t Find the HD

We have a user that added memory to his standalone- the wrong bus speed I believe. Now his HD has config errors (windows says to run scanreg /restore which we can’t run from bootable floppy. Windows says files are missing. Can’t re-install Windows- config errors on the HD. We went to boot/fdisk. Can’t find HD. How do we reformat the drive if we can not find it. NDD can’t find it either. It shows up when bios reads hardware, but can’t do a thing with it!