The laptop appears to be fully functional except for the display since it is working with an external display. Therefore, the battery is fine, the CMOS is fine, the paste is fine, the CPU isn't overheating and the pins are not shorting out from the paste, the backlight is not the problem since the computer doesn't even see the display, so there can be no video on it. In a dark room you may be able to see the backlight come on. I suspect, either a cable has come loose or the display is dead. It is possible there is a seperate drivers on the MB for the internal display and the external display. If so, then the problem could be on the MB.