if your head unit is stock, and you have had the car for a while, and you haven' t recently changed anything, and the radio just stopped working...then your tuner is probably dead!
To be sure, pull the deck and check the antenna connection. To do this, there are two small plastic flaps on each side of the radio on the front that flip open. Once opened, you use a hex key (i think 8mm) unscrew the hex screws until the radio just slides out. The antenna lead is the big black cable tv looking lead. Just check the connection and make sure it fits snuggley.