The web client appears exactly the same.
However I have now been able to access a previous version through "Note History". I apologise, but I had not previously been aware that this facility existed, otherwise I would have tried it before. Only your reference sent me scurrying to find out where this history could be.
So at least I am saved, as far as the work is concerned, even if we don't know what caused the problem.
But this is much! Thank you very much for your speedy help.