Same problem, Athens, same time with everyone else:
As I was working on Onenote 2010 (win7), my content started vanishing. I ended up with blanc workbooks. Nothing else showed. Neither sections or pages.
I then opened Onenote on two more pcs,
one running win10 and one win7.
Same thing.
I checked with my android, no problems there.
So I visited onenote.com. Everything looked fine.
I tried to "open in app" different pages (on the web). No way.
The strangest thing: I have ended up being able to 'upsync' i.e. any new material I create (i.e. a 'new section', in the app) but unable to "downsync' (from web to the app) anything.
In other words, I cannot get my Onenote app to work, anywhere.
In my win 10 laptop I have done all possible upgrades and also tried "modify" at "Apps and Features".
Nothing seems to work
I have a legal copy of onenote 2010, which suddenly failed me accross various pcs.
Is Microsoft listening ?