You mean you deleted the notes by putting them in the trash and emptying the trash? I’m afraid that will permanently delete the notes. Agenda keeps a history of your changes, and when you start syncing, the deletion will be synced to other devices.
Did you lose the data on your iPhone? If not, I suggest turning off the sync immediately in Agenda, and removing the sync data in the cloud when it asks.
If you did lose the data completely, do you have any backups? Eg a backup of your iPhone in iCloud? A time machine backup of your Mac?
As a last resort, we may be able to get the data from your Mac library, but you would have to send your whole library to use. Let me know if you want to do that.
Kind regards,
Drew