Car battery died and radio wontcome on, why?

No doubt you blew a fuse. There are many fuse box locations in the Mercedes. Under the hood, under the back seat, and in the trunk. Most owners manuals have the layout for all the fuses.

You need the security code to unlock the radio. Once the battery dies and the radio loses power it goes into anti-theft mode which requires a 4 digit code to unlock it and make it functional again. The code is usually hidden in the car on a sticker in the trunk. Or the dealer would have written it in the radio owners manual. If you can't find it , you'll probably need to provide the dealer with the vin number of the car and they'll contact Germany to get the code. You need to write it down and keep it in a safe place.

I don't no my security number i got the truck from someone i no,the radio is not coming on