Hi, I was facing the same problem after replacing the screen. While a broken backlight is of course a possible explanation, in my case it was something else.

I can not clarify completely what the problem was but after several tries of rebooting with SMC PRAM/NRAM reset, the issue vanished after opening and closing the screen a few times. Sorry for not beeing able to give clear instructions what to do. I just wanted to let you know that it is worth trying a few things before giving up. Could it be related to the Hall-sensor that recognizes whether the screen is open or closed?

Markus