Help: After repasting my screen remains black (MSI, 980M)

I repasted today my MSI GT72S notebook (980M + 6700HQ), like i did many times before.
After I finished I restarted but didnt see anything, the screen remains completely black - not even the MSI Logo while booting shows up!!
But Win10 is booted, I could check that by pressing shift 5x and it did the specific windows-sound and my keyboard is colored by the steelseries software.

I am in panic now. I even put my external monitor to DP-Port but my monitor shows "no signal"- this never happened, I always had my desktop instantly on my external monitor.

The ONLY difference while repasting was, that I put the 980M-MXM-Card out to look at it. But I put it correctly back in the MXM-Slot and put the little screw back in it and the heatsink was good too.
So did I put the MXM-Card back in the slot the wrong way? But the screws were fine, it even clicked when I put it in

Pls give me some advice.
THX!
-5754

EDIT: The "GPU"-Light on the left side of the notebook is on, which indecates that the laptop is running in dGPU-mode (but I cant switch to integrated graphics, cause I cant click on the "OK"-button......

 

I know you are insisting that you did it correctly, but from what it sounds like you took the GPU out, repasted it, and now it isn't working. You should think about taking it back apart and reseating the GPU.

But before you do that, maybe try an EC reset. Have you taken out the battery and removed AC power for ~30 seconds?

 

Hopefully you grounded yourself before handling your pride and joy.