Most likely you Photoshop preferences were reset
I understand that, as you point out, "Photoshop preferences were reset" - but just how is it being reset....I don't knowingly do it and would very much like to avoid it happening!
It's not something you should come to expect. I can't recall ever having lost Photoshop preferences spontaneously.
It says that somehow your preferences are getting corrupted or lost from the files stored in the folder(s) described here:
Once you get it set back up the way you like it, you can back up the files in those locations and as a workaround restore the backed-up files if you get a reset again.
But I'd definitely try to get to the bottom of why things being stored on your disk aren't being retained - or why Photoshop itself might be corrupting them on save so that they're thrown out next bootup.
Are you seeing any messages?
Many thanks for your most comprehensive reply which has been a great help. I've managed to restore from a backup I already had, which has saved a lot of work - thank you for that suggestion, I'd not considered it! At least I can now restore things, but in answer to your question, no, I'm not seeing any messages - my first clue that things have been reset is that I suddenly can't zoom on my mouse scroll wheel.
Perhaps I should contact Adobe if it happens again.
Once again, may thanks.