Using a flashlight hold it against the screen at a sharp angle are you able to see the faint images of your desktop icons? If you do the backlight cables as seen in this IFIXIT guide: [guide|998] in Step10. You may need to check the inverter as well.
If you don't see any icons on the desktop then the GPU logic will need to be checked. I would see if an external monitor shows an image if it does I would inspect the data cable connection to the internal display. If not the GPU board has a problem.