ST2320L large dark regions on screen

Hello,

Yesterday I came back to my computer after a few hours and the screen was much dimmer than usual, and in particular, there are large dark regions on the screen. 

I performed the diagnostics as suggested, and these spots are even more clear on the red and gray screens. I've attached photos of those two colors.

Here are the specs:

- Dell ST2320L Monitor

- MacBook Air 13", version 4,2

- Mac OSX Lion

- Onboard video card

- MiniDisplay port -> anything

- Using HDMI

- Not using a Dell cable

- Serial number: CN-0GRV8V-72872-1CK-J1LI

Thank you,

Harold