problem with my screen on bootup?

hello guys, i have an alienware 17 and lately ive been plugging it into my TV via HDMI to watch movies, anyways... now lately ive been noticing that when its not plugged into the TV and im just using it as a laptop, i boot it up and ill hear all the sounds and everything working but all i see is a black screen, then i plug it into an external source (my tv) via HDMI and then the screen works again, then i simply unplug the HDMI and everything is fine, what is causing this?