Sometimes when I open OneNote it doesn't show any of my 20+ separate notebooks and I have to reload them by double-clicking on all the *.onetoc2 files. This is a big waste of time. I keep all my notebooks in a root level folder named \OneNote, with subfolders for each different subject area, e.g. Photography. And sometimes when I do re-open notebooks this way, the application says that some tabs can't be opened.

I use Office 365 and I keep all my notebooks on my system. I DO NOT synchronize with OneDrive or any other cloud service. So what is going on here?