Have you tried jiggling the steering wheel while turning the key? I would guess yes. I think you have a failed tumbler which must be removed by cutting out the steering lock. I think this takes about four shop hours. Maybe the locksmith can tickle the tumblers and get it turned. If so, go straight to a shop and DO NOT TURN IT OFF! If it is anything like the W123's W124's, it must be in the "1" position to remove.
I hope someone else has a better idea than this. Good luck!