That said, in order to fix this, you need to do the following:
Please unplug the Cable/SAT box from power and shut the TV off.
Disconnect and reconnect the HDMI cable between them on both ends- making sure it is secure.
Plug the Cable/SAT box back into AC and once it's rebooted turn the TV on, and use the INPUT button on the original TV remote to select the correct HDMI Input.
Let me know what happens.
If you still have no luck, then postback with the brand and model of the cable box (for example: Exporer 4340HD), and also the type of cable being used to connect the two.
If you need further assistance let me know and once satisfied, please click a POSITIVE RATING, as I am an independent Tech- not a Site Employee, and the rating is the ONLY WAY I am credited.