Videos won't play - step though frame by frame

I am running Premiere Pro v 14.0 on Windows 10.

I recently completed a large video project successfully,  but a few days after I finished all the editing, something weird happened. It may have followed an automatic Windows update.

Any sequence I load from any project - I have tried several - doesn't play properly.  If the cursor is held over the play button (sithout clicking anything), the video steps through frame by frame, but won't actually play. Clicking the play button has no obvious  effect.

But I have now found that if I click the play button while it is stepping forwards (which should stop it playing), then move the cursor down so the stepping stops, I can now start and stop the video playing normally with the space bar, and only with the space bar!

I have tried uninstalling and reinstalling, to no effect.  I have tried importing working mp4's - same problem when dropped into a sequence.

Also I can't change the size of the windows when a project is loaded, though I can do so when the files are loading.

I've been using various versions of Premiere Pro since the 1990's, and I have never seen anything like this.

I should add that I use Kaspersky Total Security, which has found no viruses.  I have run two further virus checks Using Malwarebytes Pro and MSERT, but they have also found nothing.

I am assuming that either there is a problem with Windows, or I have a conflict somewhere, but have no idea where to look!

Is there a clue in further weirdness? I can play mp4's in Windows Media Player and the Windows Films & TV app, but VLC shows a variant of the "stepping through" behaviour, and looks like it is playing every 25th frame at short intervals!  From the start it won't even do that, but just cycles through the first 3 or 4 frames so moving objects just jerk back and forth.

If anyone can suggest possible remedies I will be very grateful.

Fortunately I have no video projects in the immediate future, and my computer is to be rebuilt and upgraded to Windows 11 in the next few weeks, which hopefully will fix things! But I would like both to understand the cause of the problem, and ideally fix it now!

QAny suggestions will be acknowledged and appreciated.