It may be that the circuit in the faceplate went bad.
I had a pioneer cassett w/ changer controls and it started not letting me switch between the 3. I would start the car up and either get the radio/cassette/ or changer operation and that was it.
It sucked considering I had pulled the older pioneer out to put in my other car and run that changer.
I just got fed up and bought a new pioneer cd radio but now I have my 12 disc...older version...only playing through one channel. I just have to swap the newer 6 disc w/ the older 12 disc.
The easiest way to make sure its your radio is to plug in a stock radio and see if the same thing happens.
If it does its your car and if it doesn't its the radio. Depending on how old your radio is it may be just time for a new one..