But you should still be able to push down the door lock with the door closed and you inside the car. but not when the door is open. If you're inside the car, and can't lock the door, then you have a problem. Open the door up and see what's happening in there. Hope you have been able to fix your problem since you posted.
'80 240D (sold) wish I didn't.
'85 300D (240K miles)