I had notes on my iPad. When I realized iCloud sync was not turned on for notes on the iPad, I turned it on. At that time, all of the existing iPhone notes populated on the iPad, and the iPad notes disappeared
How to return notes? There were important materials there, but now they are nowhere to be found
iPad, iPadOS 16
See >>> Delete and recover notes on iCloud.com - Apple Support
If your Notes were synchronised with iCloud Notes, they should be recoverable if accidentally deleted. If instead you were not using iCloud Notes, your Notes should have been included within your iCloud or iTunes backup.
However, be aware that iCloud backups are incremental. If your iCloud backup has been refreshed since the Notes were deleted, the deleted Notes will also have been removed from the iCloud backup - and will be unrecoverable.
Restore your iPhone, iPad or iPod touch from a backup – Apple Support
See >>> Delete and recover notes on iCloud.com - Apple Support
If your Notes were synchronised with iCloud Notes, they should be recoverable if accidentally deleted. If instead you were not using iCloud Notes, your Notes should have been included within your iCloud or iTunes backup.
However, be aware that iCloud backups are incremental. If your iCloud backup has been refreshed since the Notes were deleted, the deleted Notes will also have been removed from the iCloud backup - and will be unrecoverable.
Restore your iPhone, iPad or iPod touch from a backup – Apple Support
Notes on iPad lost after synchronize with icloud
