Dear MS_522,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, "it said to delete the app. I tried this and the notes that weren't syncing from my iPad to my Mac were lost." t seems that you deleted the OneNote app on your IOS device and your unsync notes lost.
Per your scenario, I am afraid it is not possible to restore un-synced notes, deleting the app deleted un-synced changes as well. Sorry to deliver this news. Because the un-synced notes are temporary saved in the cache file of OneNote app. If you delete the app or even signed out from the application, the notebook will be closed as well, which means the cache file could be removed as well.
Give this suggestion, the one think in my mind, you might be recover from back-end side. the best way to recover your notes from back-end side and if you are using Office 365 business subscription, please contact your admin as soon as possible to let the admin contact our Microsoft Support to request a restore. The support team over there has higher permission than us, they can also involve more resource to check from back end. This would be the most efficient way in handling this case for you.
For reference: How do I find my Microsoft 365 admin?
Also please check if you have shared the notebook with anyone else? If yes, please check if others can have this notebook .
In the meantime, to avoid this situation and avoid data loss in the further, when you encounter sync issue, we’d suggest you don’t delete the OneNote app and also do not sign out and sign in again OneNote application, please backup all notebooks before doing any actions. For example, you can copy the content and paste to document.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad
