iCloud

'If you're missing notes on your iPhone, iPad, or iPod touch' click here --> If you're missing notes on your iPhone, iPad, or iPod touch - Apple Support

If you store your notes on iCloud, check your Recently Deleted folder:

Access iCloud Drive files from your iPhone and other devices - Apple Support - https://support.apple.com/HT203052

"If you need to access a file that you deleted within the last 30 days, you can get it back from the Recently Deleted folder in the Files app, or on iCloud.com. After 30 days, your files are removed from Recently Deleted." --> iCloud Drive FAQ > How do I get back files that I deleted?

OR:

https://discussions.apple.com/thread/252543512

"See if your notes are still there on your email provider by using a web browser

If they are still there, delete your account on your iOS device and recreate it. This should recreate the Notes older.

If they are not, ask your email provider if they have a way to restore them. If they can, see step #2 to get them back on your device.

Lastly, you could perhaps recover them from a backup but note that if you do so, you will loose anything that was created after the backup."

Unfortunately iCloud is a “sync’ing” service.

When you delete something on a device; it is deleted EVERYWHERE … including iCloud.

However, did you check in your “Recently Deleted” notes folder? They’ll generally be held there for 30 days.