It’s possible that the notebook you are trying to edit was deleted or no longer exists. The following steps may help in identifying the issue and help you resolve it:
Check in Activties in OneNote if the notebook was open from there
Check if the notebook got moved to a different section or location
Check if the notebook got shared with someone else
Check if the notebook was opened from a shared link or invitation
Check if the changes are still visible when you access the notebook from a different device
Check in OneDrive if the notebook was moved to a different folder
