Hi @Henry ,
What's the version of OneNote are you using now?
To learn about the difference, please refer to "What's the difference between the OneNote versions?".
If you are using OneNote for Windows 10, please post a new question on Microsoft Answer Community for OneNote, as tag "office-onenote-client-itpro" focuses on general issues of OneNote desktop app, such as OneNote 2016.
If you are using OneNote desktop app, please firstly update it to the latest version. Then try the following to have a check.
- Have you synced OneNote to OneDrive? Please log in to OneNote online with your account to check if the notes exists. If so, please try to open them in OneNote desktop app.
- Try to restore the lost notes from backup folders. Please open the OneNote and go to File>Info, click Open Backups to check the backup folders. For detailed steps, please refer to Restore notes from a backup.
- If the above do not work, I would also suggest you try to uninstall Office completely and reinstall it to check this issue.
Any updates, please let me know.
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