Hi,
Your missing OneNote section is most likely due to sync issues, accidental deletion, or version conflicts between devices. The good news is that OneNote rarely deletes data permanently right away it usually stores deleted sections in the Notebook Recycle Bin for 60 days or keeps older versions in OneDrive Version History.
Quick Fix:
First, search for the section in OneNote.
If not found, check the Notebook Recycle Bin under History.
If still missing, restore an older version from OneDrive.
Let me know how it goes, Happy to help!