If you were using OneNote for Windows 10 then I am sorry to say the notes are stored on the system on a temp location waiting for sync to onedrive. Logging out and back in will have cleared the notes.
OneNote for Windows 10 notes content, the real place to save is OneDrive.com.
Newly written content is usually stored in the cache of the local machine.
If it is not fully synchronized to OneDrive.com deposit, and signed out the account.
OneNote clears the account's cache.
The results is you will loss all unsynchronized note content.