Lost Radio audio / Still had CD audio

I just bought a 2007 Mazda 3.  I was listening to a CD and switched to FM mode.  The radio had absolutely no sound.  Mute was not on, and turning up the volume had no effect.  I switched back to CD and the sound was fine.  Switched back to FM - no sound, FM2 - no sound, AM - no sound.  Pulled over and turned off car.  Checked to see if antenna was loose - it wasn't.  Turned car back on and radio sound had returned.  Anyone know what happened?

Shorted out I would guess. I'd just swing by the stealer and get a new one!