That indicates the video input on the TV is either not set on the proper input from the cable box, or the cable box is not outputing a video signal.
Check the cable from the TV to the cable box that it's snug. Check the video input selector on the TV that it is set for the input the cable box is connected to. Check that the cable box is actually turned on and not in standby.
If all else fails, unplug the AC for both the TV and cable box. Plug in the TV first, then the cable box, turn on the TV, then turn on the cable box.