no audio or video itunes

Please someone give me an answer to this. A couple of months ago I discovered AirPlay and was showing my family how I could play something from my iPhone to the AppleTV in my daughter's room. Since that day, when I go to iTunes on my iMac (which is where all of my movies are--legal digital copies) if I play a movie in iTunes I get a black screen--no sound or video. If I go select AirPlay and send it to the AppleTV it works fine. I have posted this in support forums before and have gotten no answers.