Is everything dark or completely black? Can you see a picture at all? Is the “darkness” uniform across the entire screen?
I would suspect that the back light is not working correctly. If the picture is there and uniformly dim, I would suspect that there is a problem with the power supply. The LEDs that provide the back light may not be fully energized. If that is the case, the power supply is faulty. If you can, post a picture of the boards inside the unit by taking off the back. That will help to confirm our suspicions.
Dan