Remove either the door rubber or the black covers near the door handle at the side of the door (after opening) and untighten the torque (or whatever) "bolt" you see in there while using a flashlight). Don't turn the bolt out to far, you only need to untighten it a little so that you can get the lock out of the door (push it to the rear after the bolts has been released a bit and then pull it out the door).
There is a pin from the lock itself to the mechanism in the door which is either broken or came loose...

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)