To better protect your important notebooks, we highly recommend that users turn on iCloud sync or perform a manual backup in the Goodnotes menu.
Suppose you have set up iCloud sync for Goodnotes documents across your devices. If one of the devices is unfortunately damaged or inaccessible, you can quickly verify the synced data on another device using the same Apple ID. To do this, open your device Settings > Apple ID (top row in the Settings list) > iCloud > Manage Account Storage. If the app is listed there with a size that looks about right for your library, e.g. a few hundred MBs or a few GBs, you have this much Goodnotes data in iCloud that you can restore from.
If you don't see "Goodnotes" listed as above, perhaps because iCloud sync wasn’t activated in the app at all, the next thing you can do is check whether you have a device backup in iCloud, as there could still be a chance that the data of your installed apps has been automatically backed up to iCloud. Just open your device Settings > your Apple ID > iCloud > iCloud Backup and verify that "Back Up This iPad/iPhone" is ON and there's a backup. If so, restore your entire device from the backup.