Dear respected Dave Monk,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
I’m sorry you’re having a problem but based on your description we can work together to resolve it.
For the situation you encountered, we deeply understand the inconvenience caused and apologize for it.
As per your description, firstly, to prevent the OneNote notes from losing, before doing any troubleshooting steps, I recommend backing up the affected notebook.
Try to close the affected notebook in OneNote app. Go to OneNote Online, open the notebook in browser first, then click "Open in Desktop app" from ribbon to open it in OneNote desktop client app. See if the error message will disappear.
If the issue still persist, then I suggest you go to OneNote Online create a new notebook, copy and paste all un-synced notebooks in OneNote Online, make sure they all appear in OneNote Online.
(I understand it will spend some time manually copying all un-synced notes to OneNote online, but this will reduce the risk of losing notes. Appreciate your understanding.) then click "Open in Desktop app" from ribbon to open it in OneNote desktop client app. See if the error message will disappear.
We appreciate your understanding that sometimes the initial suggestions may not resolve the problem very soon. However, we can work together to narrow down and resolve the situation. So, please kindly provide more information below to let us work further.
1.Does the issue happens in all notebooks or just a specific notebook?
2.Where do you save notebook? Personal OneDrive or OneDrive for business?
3.The version of your OneNote application.
Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution.
I'm sorry and I apologize for that.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad