Onenote unfiled notes missing

If you create a new Side Note, where does it go? To create a Side Note, open OneNote go to the View tab of the Ribbon and click Side Note.

Also, have you checked the notebook recycle bin?

Go to your notebook named "Personal" or "Personal (Web)". Right-click and then select "Notebook Recycle Bin".

Thanks for reply but I am trying to restore unfiled notes section not find a lost notebook

By default Unifiled notes are saved in the following location ‘C:\Users<username>\Documents\OneNote Notebooks\Personal\Unifiled Notes.one’. Browse to the location and then try to open and check if they show up in the Notebook.

If the Unifiled notes are missing from the default location, then you may check if they are present in the ‘Backup Folder’ and try restoring it. The default location for ‘Backup Folder’ is ‘C:\Users<username>\AppData\Local\Microsoft\OneNote\12.0\Backup\Personal’.

Check the below thread link which discusses about the same:

​http://answers.microsoft.com/en-us/office/forum/officeversion_other-onenote/how-to-recover-a-missing-notebook-from-onenote/b36d0eea-2f97-4f14-9924-1e02b3368389