How do I retrieve a deleted note on iPhone?

You can check to see if it is in the Recently Deleted folder. Also check to see if it was synched to your iCloud account or if it was included in a recent back-up. If in none of those places, it is probably gone for good.

Recover Deleted Texts

If iCloud is set to ON for text messages and you deleted some text messages then they are gone from iCloud so cannot retrieved! If this is the case then it is all over and no need to read further.

 If you do not use iCloud for text messages then you can restore your iPhone from a backup on your iCloud account (if you use iCloud to back up your iPhone). Once you restore then your texts will only be as current as when the back up was created. Here is ➔ How to restore your iPhone from an iCloud back up.

The other option is to restore your iPhone from a backup that you may have created on your computer. Once again, if you restore your iPhone from a computer back up then your texts will only be as current as the day and time the backup was created. 

If you have a backup created on your computer and If you choose to restore your iPhone from your iPhone from it then here is how to do it ➔ Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Axel F.