Sounds like the starter has gone bad. Check to see if you have voltage at the small wire when the key is turned to the start position. Also check for a voltage drop at the large wire on the starter. (Voltage drop = one lead of dvom at positive battery post and other lead at large post on starter.....the post, not the cable.
There should be almost no drop. If all of this checks ok, replace the starter.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."