1. Check your computer’s network connection by going to a website that you can usually access
2. Look for any error messages that appear when saving your OneNote file
3. Check your OneNote settings to make sure auto-save is enabled
4. Confirm that you have the most updated version of OneNote
5. Check the recycle bin for any deleted files
6. Try using different backups of your notes
7. If there is a significant amount of data missing, contact Microsoft Support