It could be many things. Have you had any drops, incidents with water, or anything of the sort? Googling around, the problem seems to be fairly common. I would try to get ASUS to cover it if you can go without the phone for a bit, but if that is out of the question, you can try replacing the LCD module.
If you are in the United States, this would be a fine module:
http://www.ebay.com/itm/141758646074
The LCD and glass are fused together, but it looks like there are a few different kinds of screens out there. If possible, you should open your phone up and take a picture of the ribbon cables on the inside.
I'd also recommend buying some thin double sided tape, as shown in the guide for this device here on iFixit.