The lock actuator is probably bad -- this is the part that the cylinder fits in.
You will have to remove the lock system from the car to replace it, and it is a pain in the sitting part. Costs about $65 I think. Needs to be done before it gets completely stuck and you have to grind the steel pin off....
Go to a Benz mechanic.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!