We got a new TV a month ago. Unplugged the old one, plugged in the new one, no problem. Then the screen started to "glitch" - it goes to black for a couple of seconds, then resumes. Sometimes it happens once or twice a day, other times (like now) it happens every few minutes or even less than a minute apart. Comcast phone support was useless due to language barrier (twice). We unplugged the entire setup, let the modem and router sit for a while, and hooked it all back up. That seemed to solve it, but a few days later it started again. This happens no matter what we're watching: network TV, cable (all channels), any app - Netflix, Prime, YouTube - all of them. Driving us crazy!
You need to do a few things to rule out the new televsion? Can you watch anything over the air using an antenna? Do you have an older DVR to watch a saved program?
Using an app like You Tube (found on the tv) may work if the problem is not Comcast Internet too.
adding... this sometimes happens when the HDMI connectors are not seated properly (feel/hear the click as they lock into the HDMI socket).