When you delete the app, its local data is removed as well (as the system warns you). We don’t keep any copies of user notes on our servers, so if you didn’t back up your data before this happened, there’s not much we can do, but there are still a few options you might want to try.
Check the Trash
First, start by checking the Trash within the Goodnotes app. To do this, open the Goodnotes menu located in the top-right corner of the Library view, then select the "Trash" option. This is where deleted notes and documents are temporarily stored, so you might find your missing data there.
Restore from iCloud
If your data isn't in the Trash, check if Goodnotes data is stored in iCloud. Go to device Settings > Apple Account > iCloud > Storage, and look for "Goodnotes." If it shows a size matching your library (usually hundreds of MBs or a few GBs), your data is backed up in iCloud.
After confirming this, return to the Goodnotes app. Open the Goodnotes menu again, then go to Cloud & Backup > Cloud Sync. Make sure the option "Use iCloud to Sync Documents" is enabled. Once this is turned on, your Goodnotes data from the previous iCloud sync session should begin to appear in your library. If you still do not see your notes, be sure to check the Trash again, as sometimes restored files may be placed there.
Restore from Goodnotes Cloud
If you had Goodnotes Cloud enabled, your notes were syncing continuously to Goodnotes Cloud. Even though the app was suddenly deleted, the change might not have updated in Goodnotes Cloud immediately. If you can't find your notes, make sure you're logged into the exact same Goodnotes account as before, since your notes are linked to that account and won't show up on a different one.
Recover from Auto-Backup in Goodnotes Cloud & Backup Settings
If you enabled the auto-backup feature in Goodnotes’ Cloud & Backup settings, you can recover your notes from the auto-backup cloud storage. For example, check /Apps/GoodNotes 5/GoodNotes in Dropbox if you used that cloud storage. Then, re-import your notes into Goodnotes as needed. See Import files into Goodnotes.
Restore from an iPad Backup
If other methods fail and you're okay with extra effort, try restoring your iPad from a previous device backup. Check for a suitable backup by going to device Settings > Apple Account > iCloud > iCloud Backup and noting the "Last successful backup" time. If this backup is from before you lost your Goodnotes data, restore your device to recover it.
Important Note
If you don't have a proper backup as mentioned above, Goodnotes team can't help retrieve the lost data for you.