I've had this happen on a number of different occassions. Usually turning off the engine, taking the key out, re-inserting the key and then starting the car up -- fixes it for me.

Also, when / if you get it working, use the OPT button and scroll through its functions several times and get it back to when the radio function on the LCD screen appear. I've found it could be stuck in some weird non-radio function status as well.

Good luck.