Key won't turn

A number of reasons may create this problem. First, do not try to force the key to turn. Check the battery, it may be bad or low in voltage. Key tumbler can be defective. Bad electronic ignition switch (EIS). There could be some other reasons such as defective electronic control module (ECU) or the key itself maybe damaged. Try cleaning the keyhole with a dry cloth, do not use any sort of liquid, check the fuses. Usually, the problems above need to be checked and solve by a knowledgeable person or Mercedes Benz mechanic. Also, do not allow the battery discharge completely with key in the ignition, this could damage the key or the EIS.

Reply to your posting that went directly to my email: Glad that you have learned your lesson because messing around always ends up costing money, believe me, I KNOW. Yes, post your progress, it always benefit others. You are very welcome.