Thanks for all the ideas. I tried a lot of them, and eventually got it working by going into safe mode then answering NO, which put me into the recovery program. I was able to select a date when it was last working and now it works OK again. I did notice though that the date was wrong, it had jumped three months, which was probably why it would not go to a suitable recovery date by itself. So I had a double failure to contend with and was lucky to get out of it.
Once again, thanks to all... as usual!