My Shaker 500 (2006 Ford Mustang) will not turn on. ?

I have a 2006 Mustang with a Shaker 500 audio system. The system was playing fine when I turned off the car, but when I got back in turned the car on, the audio system would not come on. There are not lights or time display showing. All of the knobs around the radio (air, hazard lights, etc are illuminated). I tried pressing the power knob to turn the radio on, but nothing happens. Is there some special on that I don't know about?