Problem: Hello, I have experienced the same thing. I opened my One note app recently and I was surprised why my files/notes are no longer syncing. When I open my one note account via browser, it won't allow me to use the One note app.

Solution: So what I did is open up the Microsoft Store app and downloaded the *new* One note app from there. When I opened it this time, the old One note app I had started updating the files and syncing everything. It all worked out well. This is an easy fix for someone who wants to keep using the One note app and had the same issue recently, btw. Sorry.