I had a bright idea today. Because of a couple of hardware error messages during booting (caused by known BIOS bugs) I thought it would be a good idea to upgrade the BIOS. I downloaded the latest version and the upgrade program, put them on a bootable floppy, and did the upgrade, making sure that I kept a copy of the original version so I could revert if necessary.
The wretched thing wouldn't boot from the hard disk any more
I reinstalled the original BIOS and still it wouldn't boot from the HD.
I dug out the OS installation CD and used it to repair the bootloader. Still it wouldn't boot from the HD.
I installed a different Linux distro on a spare partition. This wouldn't boot either.
I've fiddled with BIOS settings, installed and reinstalled BIOSes, all to no avail.
In desperation I removed the hard disk and put it in another PC, and it worked. That's what I'm using now while I ponder what to do with the other machine (the one I built a few months ago and was so proud of).
There's going to be a sign on the study wall from now on, saying "DON'T TRY TO UPDATE THE BIOS!!!"