Was the stop-down slide unlocked before you mounted the lens? If not, you might be able to unlock that and get it to come loose. another thought, did you try to fire the shutter? That may help as well. The main thing though, is the the stop-down slide(DOF preview) on the front left side of the body next to the lens. According to Canon, this must be unlocked prior to mounting a lens for just that reason. The tell is a red dot on the camera lens mount indicates it is locked and must be retracted. They also instruct to have the the film fully advanced before installing a lens. These are the only causes that come to mind.