Hi there! This usually happens when OneNote has trouble syncing. The first thing to do is check your notes at onenote.com to make sure they are safe. If they are, close the notebook in your app and then reopen it from OneNote Online using “Open in Desktop App.” If the section still won’t open, create a new section in the same notebook and move your pages there. That often fixes the problem.
Regards,
Arlene D.