As Evadora hints at, your post reads as if there's nothing wrong with your set-top box but your TV has the wrong input selected.
First, make sure that your set-top box is actually switched on.
Then, sing the remote control for your TV set (not the one for your set-top box!), press the 'Source' button. (It's sometimes labelled, 'Src', 'Input', 'In' or 'Signal' but it's probably most commonly marked by a rounded rectangle with an arrow pointing into it). That will enable you to use the up and down arrow keys to move between such options as 'DTV', 'HDMI 1', 'HDMI 2', etc, which you then select by pressing the 'Enter' key on the handset.
You need to have the correct input selected, which will depend upon which port the output from your set-top box is plugged into. (It's usually 'HDMI', 'HDMI 1' or 'HDMI 2' but if you've got an old TV and set-top box it might be 'Scart' or similar). If in doubt, simply try each input in turn until you find the one that displays the signal from your box.