Thank you everyone.
I uninstalled everything. Reinstalled everything. Did a hard reset, and it's now all working.
Hallelujah. Never again will I mess about with it!!
But in my searching for solutions, I have also worked out how to get my library borrowed books into Calibre (which I prefer). So every cloud has a silver lining and all that!
But thank you all so much for your help.
Now I'm a bit less panicked, i'm going to look round the rest of the forum.
