My 04 XJ8 has what appears to be a Security Radio Code. I was given by the former owner a card that was in my glove box that had this. I think if power is cut to the radio this must be typed in to the radio or the radio assumes it's stolen and wont work (I don't know much more then that) the other thing you may want to check is a fuse. hope this helps

 

That is correct, if the Radio is without power for a while you will need the correct factory securty code to reset it. If you don't have the security code card you will have to go the dealer to get it.