The message means that your tv and your Sky box aren't connected properly.
Turn off your Sky box and your TV at the plug and check that the cable connecting it to the TV is still securely plugged in. Check also that the cable at the TV end is correctly pushed in.
Turn your TV back on (not your Sky box). If your TV has more than 1 HDMI socket, use the TV remote to go though each of the TV's HDMI inputs to make sure you've selected the one that your Sky box is connected to.
After 5 minutes or so, turn your Sky box back on and let it start up. Check that the power LED on the front panel is green. Do you now have sound and picture from your Sky box?
