Perhaps you or someone else with access to your notebooks deleted notes accidently. Fortunately, OneNote automatically saves notes for a limited time after they have been deleted.
How to recover your deleted notes in OneNote
Open the notebook where you expected to find the lost notes.
Choose View > Deleted Notes.
If you find your deleted notes here, you can recover them by moving the pages back to their intended locations:
Tap and hold, or right-click the name of any page that you want to recover, and then choose Restore To.
Choose the notebook section where you want to move the page, and then choose Restore.
If you want to move the page to a section in another notebook, select < next to the notebook name, choose the notebook, and then select a section within it.
Repeat these steps for any other pages that you want to move out of Deleted Notes.
For more information, visit Find my lost notes.