Thanks for the responses.
https://github.com/Sigil-Ebook/Sigil/releases/tag/1.6.0
I just downloaded this. I will have to figure out how it works.
I had been using a macbook for years, reading my Books on the included app.
I got this new windows laptop and copied all my books from my mac onto an external hard drive, and then downloaded calibre on the PC, and dragged and dropped the books into calibre on this windows PC.
It was working fine, until today. I am not very knowledgeable when it comes to using PC's.