This is an excellent example of why Windows Update was a bad move on Microsoft's part. But I guess you knew that already.
The PC must have restarted with excellent timing, while you were saving.
There are a variety of services out there that claim to (try to) repair corrupt zipfiles. Try renaming (a copy of) the epub to .zip and using some of them, it
might work.
Hopefully you had a backup of the ebook.