There are several steps you can take if you need to recover a deleted or lost OneNote notebook or section.

1. First check the OneDrive and OneNote Trash. Anything deleted in the last 30 days will be able to be restored from the Trash.
2. Secondly, if a notebook had not been synced to OneDrive, and saved locally, it may be possible to restore it from File History. File History is a Windows utility which creates backups of users' files periodically.
3. Thirdly, if the notebook was located in any cloud-based service other than OneDrive, where the notebook had been enabled for version control, it may be possible to restore an older version of the notebook.
4. Finally, if none of these methods work, you will need to contact Microsoft Office tech support and ask for assistance.