Have you tried simply re-entering it? (http://icloud.com - Mail page - click the cogwheel icon at top right and choose Preferences - 'Accounts'.)
Note that you can have only three aliases on iCloud (MobileMe allowed five and if you were previously a MobileMe subscriber and had five all would have been brought over but youl would have to delete three before you could add one).
I've now come across this Apple page:
'Deleting an alias is permanent, so if you think you might want to use the alias again, you should simply turn it off. '
So I'm afraid you are stuck.