My son visited and brought a disc on which he had downloaded
How to Reset Windows 7 Password in 3 Minutes (as suggested by gpick24.)
That download didn't solve the problem. It wouldn't let the password be reset unless the 'full version' was bought.
I don't know how or what he did, but I now have a working desktop.
It took him less than an hour and it was completed by using System Restore. It seems that my user account had vanished.
When I asked what caused it he said he couldn't know for sure - 'just a glitch'.
Thanks for suggestions