Smart Cover being used with magnet in it? Or using magnetised object on iPad? Probably in contact with the smart cover lock sensor.
I think there is nothing wrong with the software and did you unplug the battery first before unplugging and plugging the LCD cable?
If you didn't then most likely the backlight circuit needs to be diagnosed. Might be a backlight filter blown.
Have you checked if the LCD is showing display with no backlight? I think you shine flashlight to check if it's showing very dim display.