There may be a plastic clip that broke off. On my son's Mustang, he has the same problem. There is a rod the connects the lock cylinder to the door lock. Put the window down and try to look at the bottom of the door with a flashlight. See if there's any loose plastic pieces there. You may want to remove the door panel if you can't see from the top. I see it's a Fiesta so I'm not sure if it's the same door trim procedure.