Heating Issues

The heat in my car just stopped working last week. The fan works fine, but it goes back and forth from blowing out hot and cold air (mostly cold air). I recently had a new stereo installed and the first time I turned the heat on that same day was when it stopped working. Before then I never had any problems with it. Also, the guy who installed the stereo told me something about the control knobs for the heat being stuck and to tap them if it stopped working. He didn't make it seem like a big deal, but now I have no heat. I just need to know if there might have already been an issue that was made worse by him removing the control panel, or if he possibly messed the heat up while installing the stereo. Thank you