I was on vacation for a month and backed-up then shut down my computer just before I left.
So home again, and having purchased a few more ebooks, I pulled up calibre .. only to find it's completely empty; just as though I had just downloaded the program. I'm baffled. I had (iirc) something over 700 books there.

Looks like I'll have to start all over - I do (hopefully) still have all the books either in the library/cloud where I bought them -lots of kindles- or in my PDF folder and/or epub folder.
There is nothing on the back-up that isn't on my computer. Any ideas??