Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
If when you turn the key to the "on" position and all the normal dash lights turn on, I suspect your front fuse box/Sam assembly under the hood on the drivers side has failed. If the normal dash lights do not light up when the key is on, then you have a failed ignition switch (EIS). It's hard to exactly what has failed without a diagnostic computer but this is from my experience. The front Sam/fuse box assembly can be replaced fairly easily. It can be found on eBay for the cheapest price. The EIS, on the other hand, is a dealer only item and has to be programmed. That part is about $500 plus another $300-400 for installation. The Sam is about $200 on eBay and you can either do it yourself, or pay a shop around $200 for installation. Let me know if you have any questions before you decide to rate me, thanks!
Oh, I'm sorry I misunderstood. If the key won't even turn, then the only two things it can be is a faulty key or bad EIS. I suggest you just tow it to the dealership if you don't have a spare key to try because if you need a new key, it takes a couple of days to arrive. Same thing with a replacement EIS because it has to be special ordered and coded. I wish I had better news but that is the only solution.
Your welcome! Do you happen to have a spare key? If not I would recommend buying one and using that for a while. If the problem doesn't happen again, then you found your problem. If the problem does happen even with the new key, then you know its an EIS and at least you have a spare key now! Please let me know if you have any more questions before you rate me, thanks!