Thanks for the suggestions. The system restore didn't work, I'd done that.

In the end, I have a back up external hard disk, got a neighbour's laptop,
and connected the two. The backed up document opened with the normal password
and I've since recovered the file to my PC.

Why I lost the password for the Excel workbook was corrupted/changed though
is beyond me. Like I say, in between I installed Norton and some Windows
updates.