OneNote Notebook has been Deleted From the Server

Ive had something similar happen to me. If you cannot find it inside your onedrive recycle bin (I couldnt), then the next step is to check if you have synced the notebook(s) in question to any other device using the onenote app.
If you have done that, its possible to create a new notebook one the device that will hopefully have a slightly older version of the notebook, then create all the new sections and move the cached version (that wont sync to onedrive) over the the new notebook (which will sync)... then you unlink the old notebook, ensure all devices can pull "down" the new notebook from the server and bobs your uncle.

Ive just had to do this tonight, painful and takes some time, but hopefully it solved your problem. I lost a little data in the process, but better than losing entirely everything.