Sorry folks, I have been busy with other things.
I will certainly look on the HDD case for the cylinders etc and see if they are still correct in the BIOS.
If these were changed would it mean that the HDD is as it was, but the PC is looking at it "wrongly" now, so doesn't recognise anything on there?
If the BIOS settings are wrong and I change them, would anything overwrite to the HDD or is it just the EPROM on the Motherboard that gets changed?
I will also try all the other advice - for which I am very grateful to you all. I have a mountain of DIY jobs, but I will get to it and report back when I do.
No, cattletruck, you are not in my bad books - just robust debate !