Inspiron 7559 black screen

Howdy I'm curious if anyone may have a clue with what's going the laptop itself boots up just fine threw a external monitor however the screen on my 7559 has stopped working it is pitch black unless I run jt threw the lcd Bist in which case it goes threw the color sequence without a  problem but then its back to black i've replaced it anyway and checked both ram sticks(could they both have gone bad?) To no avail  any help or feed back would be greatly appreciated and this is a i7 4k touch screen  model and the touch functionality still works