Is the set top box connected with HDMI or Component cables?
You might want to return to the video menu on the set top box and double check the output matches what your HDTV can accept.
On your Comcast remote press
power off and then within 2 seconds press menu
The output settings for your box will be accessable, enable or disable 1081i, 720p, 480p or 480i as necessary to work with your HDTV.
After changes, press power on to return to normal operation.
Otherwise, the set top box might be failing and will need a exchange at your local Comcast office.
Had to go to this link to get a complete guide to using my DVR setup box. It did help with some of the channels got to seup settings by this guide
use remote to power off as you said . then hit info and guide on front of box to bring up a HD setup guide.