My TV is turning on and once the white upside down V appears I get a message saying "No Signal" and then it goes dark. My DVD plays, but not the TV--no guide, nothing.
Reattach ALL your coax connections and then pull the power cord to the box and plug it back in and let it reboot.
Check that you are on the proper TV input.
As ComcastAndrew said, make sure that you are on the correct TV input. Is the box connected to the TV via HDMI or coax?