Problems unlocking driver's door.
 

I went to unlock the door on my 190E this morning and as I turned the key to unlock it I heard a little snap. Now I can't unlock the car from the driver's door. When I try to unlock the door the knob comes up about half way but won't come up far enough to unlock the door. I can lock and unlock the car perfectly fine from the passenger door and the trunk. What happened? Hopefully it's not too difficult to fix. :(

Well, I just tried working with it a little more by lubricating it and now it doesn't work at all. I can't lock it or unlock it using the driver's door, but the trunk and passenger door still lock and unlock the doors perfectly fine. :(

The 190's (W201) have a metal rod between the key lock & the door latch.

Very common for those to break OR for the retaining pin to fall out. Removal of the key lock should provide you with info on your problem.