I would assume that message is coming from your TVs tuner, and that function should be part of your TVs menu. Note: You would only be doing that if you have the Comcast cable plugged directly into your TV (not going through a Comcast box).
Tore my hair out with this one. Chatted with a tech who was very patient etc. and finally read this forum. I found the remote that came with the TV, clicked the HDMI button and Voila! the picture returned. I don't think the fix was a real lesson for everyone. My guess is that something got tampered with and the HDMI just reset the Cable box. Thanks to the forum guy. You pointed me in a new - and correct - direction.