Your TV should be switched on and out of standby mode. Press the 'Home' or 'Power' buttons to do so. This will help us to determine what you are seeing on screen.
The source is the cable your TV receives signals from, such as HDMI or SCART. Press 'input' or 'source' on the TV remote and select the source for your Virgin channels. Some Virgin remotes let you do this too.
The HDMI cable sends audio/video to your TV, so ensure it's firmly attached to the TV box. Press 'input' or 'source' on the TV remote and check you have the right input selected. For 4K TV, check the cable is connected to a 4K compatible HDMI socket.