Two possibilities
Possibly a lock cylinder problem, but more likely the plastic ignition switch actuator rod is broken. Turn to the run position and push the pin in on the bottom of the lock cylinder to remove the cylinder. With the cylinder removed use a larger flat head screw driver to check the steering column operation. Try to start the car and shut it off with the screw driver. If it starts and shuts off normally, you need a lock cylinder. If not, and it still wont turn to the off position, then the previously mention plastic ignition switch actuator is broken and the steering column will need to be overhauled to fix. If this is the case you can temporarily drop the ignition switch down off the column (two t-27 screws) and manually overide the steering column till you get it fixed properly...Dave