If its one of EA's games then ya its quite likely the key got stolen by someone else, unfortunatly EA has had a leak for the last several years of their key generation systems for virtualy all their games.
But ya, do as Packerton suggests first (welcome to subsim Packerton btw
)
Failing that you could try EA tech support
here are some support links for battle of middle earth
Problems Logging into my Account
Error: 'Serial key already in use'
Serial Number Replacement