From "it's now hooked up to the Sky system from the lounge", I assume that you've got some type of Sky equipment plugged into an HDMI or Scart socket on your TV.
First, make sure that device is definitely plugged in and switched on!
Then press the 'Input' button on the TV's remote control. Use the 'Up' and 'Down' arrows (in the central circular bit of the remote control) to go to the relevant source. If you're using a Scart socket it will either be 'AV1' or 'AV2'. For an HDMI connection you obviously need to select 'HDMI'. In the unlikely chance that your external device is connected by 'phono plugs', into separate sockets, you'll need 'Composite'. Once you've highlighted the most likely source, press 'Enter' (in the middle of the circle) within 6 seconds. If you've selected the correct source the output from your external device will now be displayed; if not, press 'Input' again and try a different source.
Chris