Here's what happened, (don't laugh
)
The PC clock battery had gone flat, so the PC did not know the date etc. I replaced the clock IC, (with internal battery) and went into the DOS to reset the date and time.
While I was there I was looking at the HDD settings in the BIOS and was trying to see how the hard disk was configured so I could configure a second HDD to be a back-up. I didn't know enough about the BIOS or what I was doing, so I backed out, but unfortunately when the message came up "save settings and exit" or "ignore changes and exit" , I accidentally, stupidly clicked the wrong one !!!!!! Aaaaaaargh !!!!
The PC was still fine and worked, but after I had shut it down and next switched it on, it said it couldn't find Windows, only DOS came up. My computer expert told me I had erased the Boot Head File, but I don't know.
I have to get my PC out of the loft and see what HDD it has - the caddys I have found so far accept SATA HDDs, and the connector looks different to the one I remember on my HDD.