Same bizarre thing happened to my 95 E300d. I lent the car to a friend and she called from the store to say the ignition was stuck. Iwent over and sure enough--no way to turn it. somehow by moving the steering wheel and the shifer and turning the key-make sure the key is all the way in, it worked. It has never done it since. Are your wheels straight ahead and is the steering lock engaged? Try turning the key while turning the wheel free of the lock point. The wheel should turn a little bit in one direction, but not in the other if it is jammed against the lock.
Sorry for the unscientific response. Hope it helps.