Lukas Guy, just to reiterate, here is what Apple suggest:"
Blank / No Video, No Backlight
• No video
• No backlight
• Dim backlight
1. Verify display being used with supported system.
2. Verify USB/display/power connectors are fully seated.
3. Use with known-good system. If used as second display, check display preferences to see if display is recognized by system.
4. Check brightness setting.
Check Result Action Code
1. Verify display’s USB hub and built-in camera are listed in the System Profiler’s USB device tree.
Yes Power supply and USB communication OK.
Go to step 3.
No Go to step 2.
2. Unplug and replug the Mini DisplayPort connector into a known-good, supported,
powered-up portable system and monitor the portable’s display. Verify that the portable’s display briefly turns off then back on.
Yes Display detected by system.
Go to step 3.
No Go to Dead Unit/No Power symptom code flow.
3. Darken room and connect to a known-good supported system. Verify backlight by looking for faint glow from display.
Yes Video signal from host system OK. Backlight ON.
Go to step 5.
No Go to step 4.
4. Verify that the LCD function interface cable and LED driver cable connections are secure. See Functional Overview.
Yes If connections are OK and secure and the display is still blank, go to step 5.
No If cable is damaged, replace all-in-one cable or replace function cable.
5. Shine bright (low heat) flashlight into the front of the LCD. Verify if an image is being displayed.
Yes Image present but backlight is not ON. Go to step 6.
No Replace LCD panel.
6. Verify voltage on logic board between test point TP81 (24.5VDC) and chassis ground
(GND) is 23.3–25.7 VDC.
Yes LED backlight power present.
Replace LCD panel.
No Poor or no LED backlight power at logic board.
Replace logic board."
Step 6 is most probably the give away. If all this checks out, you will have to replace the LCD. Hope this helps, good luck.