To better understand your issue, we would recommend replying here with a photo of your TV screen. Please let us know as well the model of your Roku device/TV which can be located by navigating to Settings > System > About.
With more information, we will be able to assist you further.
The video cable is not connected properly or the cable is faulty
Check that the video cable is firmly connected to the back of your Roku player and to the HDMI or composite connector on the TV. If you have a Roku Streaming Stick, then make sure it is firmly connected to the HDMI connector on your TV.
If you can, check to see if you have a faulty cable by connecting your Roku player with a different cable.
There is a problem with the TV
If you can, try connecting the Roku player to a different input on the TV to see if the problem is with the connector on the TV.
If you can, try connecting the Roku player to a different television to see if the problem is with your Roku player or the TV.
If all else fails…
Disconnect and reconnect all cables including the power cable.