Hard drive failures are common. They run on a continuously. But first I would make sure the CMOS is set correctly for the drive you have. Secondly, if you have a I/O board that it has not failed you. Also I have seen viruses destroy data on hard drives.
There is nothing about replacing a hard drive in the maintenance manual.
Assuming it is a dead hard drive: You need a new one. It has to be a PATA or an IDE type, NOT a SATA. The smaller in capacity, the better as you will FDISK it to only 528MB which is what your old BIOS on the mother board will support.
Then you will load DOS 6.20 onto it, and then the machine software. Lastly set your tool change height, orient position, spindle drift, spindle gain, backlash, etc.
If you know a "geek", this would be a good time to buy him/her dinner and ask for help. This is not rocket science but you have to be PC literate to do.
Another option would be to get a LOADED DOM (disk on a module) from EMI. This is a solid state drive that mimics a hard drive. You plug it into the IDE port. You would still need a current back up of your parameter file (bmdcprms.sys) or the former applies to set TC height, orient, etc.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)