Hi, Tom Cunningham1
Welcome to Microsoft Community.
I understand that your OneNote application on Windows has a “File may be corrupted” message.
You are experiencing a recent hot issue. Recently, the community forums have received feedback from users in different regions that OneNote is having file synchronization issues, or missing files.
In both threads, Microsoft's OneNote team manager noted that this is a newly discovered issue that is currently being actively fixed:
OneNote desktop app cannot sync with OneDrive - Microsoft Community
OneNote sync failure error code:0xE00009C8 bb0ur - Microsoft Community
Thank you for your understanding. For the time being, I would recommend that you use the web version of OneNote as a temporary alternative, as mentioned by the OneNote product manager.
Since Microsoft Community is only a technical discussion community, it cannot participate in development-related content. This requires the development team to further improve the product's functionality. Thank you for your feedback.
Best Regards,
Mila.L-MSFT| Microsoft Community Support Specialist