There are a 3 things that I can think of that could be wrong:
1. The brightness is set to the darkest setting. No idea why iOS does this, but sometimes after a screen repair iOS will turn the brightness all the way down. The fix is as simple as going into settings and turning it back up.
2. The LCD isn't plugged in all the way. It happens.
3. A backlight filter is blown. This is extremely common with iPads since you can't disconnect the battery before taking the screen off. One misstep connecting the LCD or using metal tools to disconnect the digi and that filter can and will blow.