It seems like a problem on either LCD cord or LCD itself. You can try to change the LCD cable then LCD if you are familier with it. As you said, your display is gone on the laptop screen and you can see dim on the external monitor (HDTV). You have two options, one is to be fixed by computer repaire stores (BestBuy,Microcenter,computer geeks etc), second, you could try by yourself by googling it. I am pretty sure it's either LCD cable or LCD itself. We had many problems like yours and the company either replaced the LCD cable or LCD itselt.