I can think of several things to try. If you are using a cable or satellite box, unplug it for a minute. Then plug back in, wait for it to reboot. Also, try the same with the tv.
Also, maybe someone had used the tv beforehand, and played with the settings on the tv or cable/satellite box. Are you using just an HDMI cable for audio and video, or seperate cables for audio and video ? Are the cables plugged in all the way ?