Sorted !!
Once again thanks to all who chipped in with advice. Not sure about all the steps but I think I followed the Nvidia link and updated the driver. Although the attempted download merely said I had already got the best one for this computer, nevertheless when I closed everything down and restarted, back it came full screen. Well minimised screen actually, but all I had to do then was choose full screen and that is what I now have again.
So what caused it? One of those very annoying updates which sometimes keep me waiting nearly half an hour before I can get switched on? What use are they anyway? Should I stop them, and how?
On my old PC running Vista I am constantly bombarded with urgings to update. Once when I gave in and tried, it didnāt work, so I ignore it now and that old PC, which must date from the very inception of Vista, works perfectly and never gives any trouble. 
I can reverse 3 40 foot trailers round a blind corner but Iām buggered if I will ever understand computers. 
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Windows 10 - so not easily.
You can try setting your network connection as āmeteredā, if you have W10 Pro there are some settings that allow you to defer updates for a bit.
But ultimately Microsoft want everyoneās computer to have all relevant security updates and stuff (or retain complete control over your hardware depending on your POV).
As I suspected, I think Iāll pass on that and live with it. 
But,
I really shouldnāt have said that. I had left it chugging away uploading my latest video to You Tube. After I got back to it it had switched off only 21% done. Then I had a whole lot of plugging and unplugging, freezing, various external drives and the thing that loads the video from the cameraās card before I decided that it would be quicker to use the video editor because it first saves to file and then uploads quicker. So that then froze. So through the whole performance yet again.
I really must remember never to praise anything again. 
The long term solution David is to never buy a Windoz machine ever again. I made that decision ten years ago I havenāt looked back. I use Office 365, that OK but MS have never built a decent OS and have certainly built a few dogs, Vista and W8 come to mindā¦
Well if I live long enough to buy another Iāll look around first but getting my head around something new has always been a bugbear for me.
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Yes, I had to sort out a palās PC a few weeks ago and Iād forgotten all my Windows stuff. It start to come back and then I thought, sod it, and installed Linux. Sheās delighted with herself.
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