Retrieve deleted notes

Have you checked Notes for a Recently Deleted folder? If you were syncing Notes with iCloud, try going to iCloud.com/Notes using a computer browser to see if there is a Recently Deleted folder.

If you are using a device and you aren't syncing with iCloud, restore the notes from your backup.

Backup - Restore your device from an iCloud or iTunes backup

If a computer, do you have a backup you can restore from?