
@Nguyen, Tee
Is this user using OneNote 2016?
Please let this user try to find the lost notes first, such as check Quick Notes mentioned in article.
This user could also try to restore notes from a backup.
If the steps do not work, it seems the notebooks haven't been synced to OneNote for the web(OneNote online) because of Internet connection or the notebook-self.
According to the error message shown on OneDrive Online, please refer to the following article.
OneNote error: "Content Not Yet Available"
- Opened and accessed across different devices. Check whether the content is visible on another device.
- Shared with multiple users. Check whether other users experience the same problem.
If "no" (content is visible), move the available content to a new section.
If "yes" (content isn't visible), consider the next option. - If notebooks stored in OneDrive for Business. Follow the steps in "Restore your OneDrive to restore the content to a previous state".
Hope the information could be helpful.
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