that has happened to me and my car 4 times, all i do is get the spare key, spray wd40 and get the attachment tip on there and spray in there then get pliers on the spare key and then gently give force, my idea is if you get it unstuck keep it lubed up, the last this happened to me was 4 months ago and its turning smooth.

some advice is to get a brand new cut key from your mercedes benz dealer ship, i got a new one and i was shocked... how does the key turn haha, it was so worn out.