When your television and Comcast cable receiver box are turned on, a blank screen is a sign that your television is not receiving a video signal from your set-top receiver box or from Comcast. Start troubleshooting this issue by inspecting your coaxial cables for damage, but don't rule out bad equipment as the source of the problem.
Cables and Connections
Comcast's signal arrives in your home via coaxial cables. Any damage to these cables -- such as crimped or frayed cables -- can degrade the quality of the image and audio you see and hear on your television. Additionally, the ports where these cables link to your television, receiver box and even wall outlet must be correctly and securely connected to ensure a proper signal.
If your blank screen only happens on high-definition, or HD, channels, the problem could originate with a different kind of cable. While a coaxial cable carries the signal from Comcast to your home, an HDMI cable transmits HD signals from your HD receiver box to your HDTV. Damage to your HDMI cable may prevent you from seeing a clear picture -- or any picture at all -- but only when you're watching HD programming.
Your Comcast cable set-top receiver box is a gatekeeper between you and Comcast. This box prevents you from viewing channels and features for which you haven't paid, while ensuring you receive the channels and features to which you have subscribed. If you haven't subscribed to a channel or a tier of channels, you can still see the channels on your onscreen programming guide. However, when you click on them, all you see is a blank screen or a screen with a message reminding you that you have not subscribed to that channel. Your television must also be tuned to channel 3 or 4 in order to broadcast cable TV programming.
Problems with your television or receiver box can also lead to a blank screen. For example, a burnt out backlight on an LCD TV turns an otherwise bright screen into a dark screen; replace the backlight to resolve this problem. Your cable box may also not fully or correctly install updates from Comcast; however, you can force your receiver box to search for the latest updates from Comcast by resetting it: unplug your Comcast receiver box from its power source for 15 to 30 seconds, then plug it back in and it will automatically search for updates.