This is a rare issue, and we're sorry you've encountered it while working on your book. We are working to fix it, but in the meantime, we recommend the hacky workaround set out below. We're sorry for not having a better answer to this issue yet.
1. Back up your work. If you have previewed or published in the past, you can find a backup of your work here:
2. Switch to another writing mode here:
3. On the same page, switch back to the In-Browser writing mode
That should work, and shouldn't take too long to do - we're sorry we don't have a process that's as easy as clicking a button.
If that does not work, the best thing to do is just to set up a new book. If you're on a Free plan, please let us know you had to do this, and we'll reset your previews & publishes to zero. We can also help you get back the URL of your original book.