I have never worked on an ignition key. Based on the posts above, I theorize that there are two reasons for the key not turning:
(1) the pins inside the tumbler worn out, or
(2) the steering lock jammed.

Can anyone confirm my theory? If the problem is caused by a jammed steering lock, change the tumbler, as indicated by unkl300d, will not help. Is it easy to replace the the steering lock on 300D W123? I tried to search this forum for "steering lock" and found more than 300 topics. I did not have time to go thru each one of them.

David