Volvo: Radio Code, volvo repair, volvo wagon

Question
I have a 98 Volvo Wagon. The parking lights were left on and the battery ran down. When everything came back up, the radio was asking for the code. My teenage son who thinks he can fix everything, typed in what he thought was the correct code. Now the radio is reading "OFF". We called the Volvo repair shop and they told us to take the battery back down with the radio off. Supposedly when the battery was charged it would ask for the code again. NOT, it still says "OFF". Any help or suggestions would be appreciated.

Answer
Linda,
Leave the key in the accesory position and the radio on, in about two hours it will ask for code. Type in the correct code and the radio will come back on. The code is provided to the owner at the time of purchase. There is a credit card size card that will have the code. If all that fails call a dealer and provide them the VIN number the radio serial, part and model number so they can get the code  via the Volvo data base.