Hi Tom:
Sorry to have taken up your time, and many thanks for your help and advice. I
*had* entirely forgotten how to initiate the checkdisk procedure so your step by step guide was much appreciated! (I've cut it out and kept it for future reference.)
Because of my own post-stroke memory problems -- rather than the computer's -- I am finding things to do and procedures to follow much more difficult than I used to, and the result of that is a rising frustration (with myself) that doesn't help at all. So.. . . having forgotten how to initiate checkdisk and hesitant about continuing to blunder along, I decided to take the risk of trusting to Tweaking.com's repair program, and running "all repairs" in Safe mode. There were some optional preliminary steps to pursue including a complete power down, but I skipped those because having just suffered the lengthy frustration of the PC failing to even launch Windows, I figured I'd best run the repair program whilst such was still possible -- admittedly, I did, however, tick the check box to reboot to Safe mode with networking.
The reboot worked fine and I left the program to it. About 90 minutes later, I returned to the PC and there were no warning screens on display so I (again) trusted to luck (and the brilliance of unknown others) and went to Start / restart. And yay!! It worked!! The computer re-started, no hesitation at all, Windows was up and running in under 30 seconds. (I have the OS installed on an internal SSD.)
Meantime, the repair shop had been back in touch, it's a lovely, friendly, response little firm which although we're in the middle of lockdown is doing what it can to offer a value for money emergency service, and it offered to look at the PC as soon as I could drop it off.
That hasn't proved necessary, because all is cured. Or rather, I think it is. I'll post a separate query in regard to the other problem.
Once again, then, many thanks for your help and sincere thanks to all those involved in the writing of the latest version 4 of the Windows Repair program. My PC is shutting down as it should now and booting up as it should, too -- can't do better than that!