Radio code?

Whats up people. I helped out my father today by putting in a new battery in his 2000 Honda Accord EX today. After installing it, we started the car and now the radio wont work. It spit out the CD that was in the player and now the display says "code". What's the code? Where can I get this code? Obviously this is the factory radio and I believe the keyless entry is in the radio head. My dad is pissed since he cant get his dose of talk radio. Any help would be apreciated! Thanks guys. Ohh BTW, he bought the car factoty pre-owned from the dealer. No manual.