Jump start problem C55
 

Battery died on my C55, hooked up a battery charger and by mistake, hooked both leads to the positive side of the battery, car will crank, but won't fire, when I release the key the starter continues to turn, why have I blown?

If you turn the key to OFF and it continues to crank, then you have a big problem. I suspect you mean you just let go of the key and it continue to crank, which is the "tipstart" trying to start, so in other words this is normal. That said I have no idea why it won't start. The car has roadside assistance, you could have called them for a jump start for free. I'd call them now and explain the situation and maybe they can send a tech out to look.
Gilly

If you have hooked both leads of your charger to one side of the battery then, if anything, you have blown your charger. The car wouldn't know the difference. But wouldn't it be kind of obvious if both clamps of the charger are on the same side of the battery?

You would think, I placed the + side of the charger on the Fuseable link cap, then the - on the + side of the battery. I