Recovering a lost note from my device

If the note vanished on your iPhone, the first thing to do is check whether it actually synced somewhere instead of being deleted. Log into iCloud.com < Notes and see if it’s sitting in Recently Deleted, you’ve got about 30 days before iCloud wipes it.

If Notes wasn’t syncing to iCloud, the only other real recovery path is a backup, an iCloud device backup from before the note disappeared, or a Time Machine backup if you also use a Mac that syncs Notes. Just be aware restoring an older backup replaces current data, so make sure it’s worth it. Outside of iCloud or backups, Notes data isn’t stored in a way you can manually pull back on iOS.

you can check if it's still in icloud by login to icloud.com using a browser

otherwise you can use timemachine if you have set that up

Back up your Mac with Time Machine – Apple Support (UK)