Hi @Alex S
The Q&A forum currently supports general usage issues of the OneNote Desktop app.
If you have any questions about the OneNote UWP application installed from MS Store, it is recommended to post a new thread in Microsoft Answers Community --- OneNote.
Besides, please note, Microsoft Support for Office 2013 has been ended on April 11, 2023.
Thanks for your understanding.
When I used OneNote in a web browser, all the data is still there.
I personally suggest you close the notebook include the section groups and re-open it (Before closing and reopening notebooks, make sure you copy and back up any unsynced recent data on the originating PC or device.).
- Right-click the notebook name in OneNote sidebar, close it.
- When the notebook is opened in OneNote for the web, select Open in Desktop App on the ribbon to re-open the notebook in the OneNote desktop app.
I also suggest you try the following workaround:
- In the OneNote desktop app, create a new section within the same notebook.
- Copy (but don't move) all of the pages from the old section into the new one that you just created. You can do this by right clicking each page, and then selecting Move or Copy.
- When all of the pages have been copied to the new section, press Shift + F9 to manually sync the notebook. If a progress bar appears, wait for it to finish.
For more, please check in this link: Fix issues when you can't sync OneNote
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