You'll have to take it to the dealer, who will then remove the radio to get a code off of the radio. They will contact the factory and enter the code. It's part of the anti-theft system. After talking to a guy in the service department, I went by the dealers and the guy at the parts counter happened to have a sheet of codes that he had copied at no charge. He started to put the codes in one at a time. Finally, he got the radio to work. He did this at no charge to me. The guy in the service dept. wanted $45 to do the same thing.
when you bought your car, they (dealership) should have given you a code to unlock your car. It also came with your owner's manual. Check that first to see if you still have the card that has the 4 digit code for the radio. If you do not have that, then talk to your dealership and they might have to replace the whole radio system and give you a new code for to unlock your radio in the future.