Why does my stereo not turn on? Any ideas?

I left my lights on and a guy helped my by trying to charge my battery. He accidentally placed the positive cable on the negative on my battery and sparks flew. I got the car running but my radio has not worked since. It doesn't even turn on and show "safe." I checked the fuse for the radio in the box under the steering wheel and it was not blown. I have heard that there may be a fuse behind the head unit. Is this true? This happened two weeks ago and since then, everything else in the car works fine. The lighter adapter works.