Calibre won't open my ebooks

[Perplexed1]

Hi all

I fiddled with my font size and type in Calibre today via Preferences. I had opened ebooks prior to that today and all was OK pre doing this.

I am now unable to open my library/individual books from the calibe 64bit ebook management link on my taskbar. When I try this my monitor flashes briefly on the left hand side of the screen and it looks like the app is trying to open but does not.

I googled the issue and tried the following, none of which worked - shut down and restarted the PC (Win 10), closed Calibre on Task Manager, uninstalled and reinstalled the most recent version of Calibre 8.12.0 which I had installed this morning, uninstalled this version and installed version 7.26 as it's supposed to work with Win 10. Googling suggests clearing the cache but 7.26 directories don't show a cache I assume because it's a new install. I also tried repair option?? on Calibre but nothing happened.

I found a thread on reddit which was about using File Explorer and right clicking on an EPUB. From the dropdown I then select open with Calibre ebook viewer and that works. The ebook viewer actually opens the book.

I went to Task Manager and it has two separate lines for the Calibre ebook viewer - one with two books and one with three books. I assume the three books are ones I tried to unsuccessfully open pre reading the Reddit forum as I only have two books open via the Reddit suggestion. There is also two lots of Calibre ebook management lines showing on Task Manager. I end task on all of them, reopen Calibre e-book management, go back to Task Manager and there are three lines one each titled Calibre Library (the one I was trying to open), one for Calibre worker process and one for Calibre Window Host. I'm not sure whether this is unusual or not. I then went back using the reddit process to open a book. I can do this and what shows in Task Manager is the book plus a line "Qt QT web engine process".

So I can open individual books, but I need to know how to get my libraries back. Help appreciated. Thanks all.