I had that happen to me a while back. I was about to try a pair of pliers on the the thing when an old man walk up to me and said, just pull the key out a 1/16th of an inch and it will turn. Sure enough when I backed the key out slightly it turned. I had to remember to do that for weeks till I could replace the lock cylinder. Later I found out this is a common problem with Ford locks.
Hope that helps!