Hello Yesenia Perez3,
Welcome to the Microsoft Community.
Recently many users in the community have reported encountering 404 errors in OneDrive, where OneNote is usually saved, I presume your issue may be related to an error in OneDrive, we have reported the issue to the OneDrive team, who are aware of the issue and are looking for a solution. Please be assured that your data will not be lost.
Please be patient and I will keep you informed if there are further updates on this issue.
Thank you for your understanding and patience.
Best regards
Miyeon.S - MSFT |Microsoft Community Support Specialist