Couple of things, i can think of.
sometime there is a key eg. <Fn> <F6> to switch to an external display does this do any thing?
the Graphics drivers - do they see the new display?
Is the Cable HDMI at both end's? eg. DVI > HDMI.
Does the Cable work if you plug it into a DVD player and the same TV?
Does the Laptop need to have a certain HDMI version to send the data to the TV?
Are you running the resolution of the graphics card at a compatible resolution for the TV?
Can you supply the Thread with the exact make and model of both the TV and the Laptop?