could it be that your tv is just very very dim? this is the case if you have a burned out backlight.
if it turns on and you can see text and tv functions but no video then perhaps an issue with the port/board which controls that video function.
if you get nothing at all it could be a short inside.
you could try unplugging the tv for a short period of time and holding the power button for a reset (depends on your model if this acts as a reset)