Going to need more info than that to help you out buddy. Try pulling OBD codes as a start, you can go to Autozone and they will lend you the tool to pull codes for free. Second what did the dealership originally say was the problem (since it obviously wasn't the issue this will help us eliminate a variable). When it won't start we need to know what's missing...fuel or spark (more than likely not air or compression).

Might want to pm HDDP, this could be the anti-theft going wonky on you...I'm not sure exactly how it works, but I know the key is mated to the car. This may sound stupid, but it'll be a $.50 try, go ahead an replace the battery in your key.

And also, call a Mercedes dealership and talk to the service guy there and ask if they have heard of an SLK320 doing this (that's what our car is).