Generally, if you can't find some of your OneNote notes, it doesn't mean they're gone. Notes can end up in somewhere.
Have you synced notes in OneNote to OneDrive?
Please sign in to OneNote online, could you find the lost notes?
Please try methods in this "Find my lost notes", they may help you find your lost notes.
Besides, go to File>Info, click Open Backups , and then open the folder and section that contains the lost notes to see if there are your lost notes in it.
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