1. Try turning the TV off with the switch on the TV itself.
2. Push the TV button on the Comcast remote and then the Power button.
3. Use the remote that came with the TV itself and push the Power button.
4. Put new batteries in the Comcast remote.
5. If all else fails, unplug the TV, unplug the cable box, wait a minute or so and plug everything back in and start over from fresh.