Try going into setting and then screen or display or brightness and turning automatic brightness off.
Also, has the brightness been decreasing over time or did it happen at once. That would either validate or disprove @dadibrokeit 's answer.
Here's one thing to try.
1) Shine a torch at the ++display++. Check if you can see stuff in it. This will test the lcd screen theory
2) Shine a torch at the ++camera/faceid++. Check if the brightness goes up. This will test the camera+automatic brightness theory.
Let me know how it goes.
Falcon16