Hi,
First thing you might want to check is the brightness setting of your laptop, turn it all the way and see if you'll get any change. If that didn't fix the problem there's a problem with the back light of the LCD screen. You need to open the laptop and check if the cable that connects the LCD back light is seated properly because if it is you'll need to replace the entire screen with the exact model because you cannot just buy the back light for it.