Hello, I have a problem with my imac. what have I done: I have a imac 27" late 2009. I had the famous white screen, so my videocard was broken. I ordered a new one. installed that one, imac works fine. I reïnstalled with Sierra and tested the imac for about a week. Today I decided that everything works fine so I want to screw back the imac together (some screws inside where not screwed back, so I screwed them together. Connected back the lcd. But when I start now the imac I have a black screen on my imac. External monitor works fine but nothing on the 27" screen. I see the ligt of the imac goes on. When I check in system preferences I see the 27" monitor, I can adjust the brightness (I see the black go more or less dark when I slide the slider). But nothing on the screen. I have the same imac from my wife. I tested it out with another screen, same result. I placed back the broken graphics card, but not a white screen, but same black screen. Anyone an idea what the problem can be? The connector on the logic board looks ok, nothing special. smc & nvram allready done, doesn't help. Any idea?