I have Dell Latitude D520 (for 3 years)
and my screen is dark.
I try to set to maximum and still dark.
Is the screen broken? Is it fixable?
Sounds like a hardware issue. Thankfully replacing the screen is not very hard and the should be fairly cheap. I recently had to replace one on a latitude e6400 the screen cost $115 direct from Dell and it took me all of 20 minutes to replace. I have replaced the RAM on many D series Latitudes which involves removing the the area around the screen and the keyboard. It is a very simple process just make sure you don't use a metal screwdriver (it will scratch) to pry the front plate off and also be careful with the keyboard. Other than that follow this service manual and you will be fine.