I don't have my Asus laptop in front of me but there is a function key that you can use to alternate the signal between the external monitor and the laptop screen. It's indicated with two rounded-edged rectangles, one set slightly aside from the next. See if that toggle switch will take care of your problem.
Also, if you're proficient in taking things apart you may want to look at some youtube videos on how to replace your laptop screen and see if you feel comfortable doing it.