A:Answer Without knowing too many details from your question, and not to insult your intelligence, I'll start with the basics.
1) Did you connect an HDMI cable from the dock to your TV? There has to be a physical connection to your screen through HDMI.
2) If it is connected through HDMI, did you select the correct HDMI input source on your TV? Unless you're using a stereo receiver or HDMI switching device, then you'll have multiple inputs on the TV to choose from, should be labeled on the TV where you plugged in as "HDMI 1," "HDMI 2," etc.
3) One thing I noticed when I connected mine is that if the Switch is in sleep mode, nothing will appear on the TV. I usually press the Home button on a controller to wake the Switch up and then it will display.
4) A final check would be to try a different HDMI cable before giving up.
Hope this helps,