When your TV and set top box are both powered on but your TV is displaying a message that there is no signal, this is an input issue with your TV. To resolve this, you simply need to switch your TVs input to the source that your set top box is plugged into.
Most set top boxes are connected using an HDMI cable, so you will want to switch your TV to the HDMI input.
If your set top box is connected using RCA cables instead of an HDMI cord, you will likely select the input on your TV called “Component”.
If your TV’s inputs do not include these specific options, you can cycle through each input to see which corresponds to your set top box’s connection. As you cycle through input selections, leave the TV on each input for a few seconds to allow any connection to come through on your screen before moving to the next input selection.
For specific information that pertains to your particular TV, be sure to check the manual that goes with your TV. You can usually find television manuals on the manufacturer’s website.