There's no UnDo for rules. You'd have to reverse it manually or create
another rule that reverses things then run it manually on the folder(s)
where messages were sent
Deleting a rule doesn't undo its previous actions, and that is what the OP
asked. As Vince said, NowU will have to either move the items back
manually, if they still exist, or define a rule that reverses the action of
the previously executed rule (which is not always possible). If NowU does
backups (yeah, I know, not likely), he/she can retrieve an old .pst file
from there.