Comcast cable boxes translate digital signals from the cable company into broadcast media that displays on a television set. Because of the hardware, software or cable connection, your cable box could have an error that prevents it from properly unscrambling the signal, resulting in a distorted or completely absent picture. Luckily it is very easy to reset your cable box.
Make sure your Comcast cable box is turned on and plugged in. If the box is off, unplugging it will not forcibly disconnect the service from the cable provider.
Disconnect the cable box's electrical plug from the wall socket. This will wipe all channel and directory settings, but not previously programed information saved in the box's hard drive and memory.
Wait one minute, then plug the cable box back into the wall socket. The cable box will now power back on and re-download program information over the cable connection.
If the power cycle did not work to fix your scrambled screen problem you can call Comcast Customer support at (800) 266-2278. Ask the customer service representative to reset your cable box from the Comcast Computer Network.