This problem is quite common on pre 1985 Mercedes, especially if they have been left outside in the rain for long periods of time. The rain gets down into the doors creating excessive moisture which, over time, corrodes the door latch and door catch assemblies. The door literally becomes stuck and no amount of pushing the knob or pulling on the handle will open the door.
Problem & Solution
Common among these chassis:
Your first objective should be to try to get some rust penetrant / lubricant sprayed onto the door latch assembly which is located inside the upper rear of the door. When the door is closed it is near impossible to remove the door panel without damaging it. Try to loosen the fasteners and pull back the panel far enough so you can spray the penetrate towards the latch using an extension tip on the spray can as shown in the picture below:
Work the door opening and door locking rods back and forth as you spray the lubricant onto the latch assembly. If possible, try to let the lubricant sit overnight. If you have a door that has enough gap, spray penetrant through the outside crack onto the latching center pin and striker assembly. Have someone get inside the car and pull on the inside door handle while you pull on the outside handle (either with the button pushed in or the handle pulled out to release the catch). Don't use excessive force when pulling on the outside door handle. If you are not careful, you can can break or damage it. If the door still won't open have your helper inside put his shoulder into the door to force it outward. This is how we got the door open on this 110 chassis sedan. Look at the corrosion on the centering pin and door striker in the pictures below. They were literally rusted together. It took a heavy shoulder bump from inside to get it to pop open.
If the door still will not open after attempting the above, you may have to cut the door panel off the door and use any mechanical means necessary to release the latch assembly from inside the door. In a few cases I have ended up destroying and having to replace both the door panel and the latching mechanism.
Once you successfully get the door open you will need to clean the parts and lube them with proper lubricant. See the special lubes under related products.