I deleted data from a folder in Notes. Can I get that data back?

Hopefully the note was deleted less than 30 days ago. Try the following:

Open the Notes App

Go to the Folders list

Tap on the Recently Deleted Folder

Tap to select the note that you want

Tap on Move and choose a folder to restore the note

After 30 days, a note that has been deleted will no longer remain in the Recently Deleted Folder

If you deleted the content inside a note rather than deleting the whole note, Recently Deleted won’t help because the note itself still exists.

First try Edit > Undo immediately if the deletion just happened. If it’s already synced to iCloud, check Notes on iCloud.com and any other Apple device that may not have synced yet. If the deleted text was replaced/removed earlier and has already synced everywhere, recovery becomes much harder because Notes doesn’t keep a normal version history.

In that case, your best chance is restoring from an older iPhone/iCloud backup, but only if that backup predates the deletion and the note was included in that backup.