A tip for knowing when your device is powered:
Activate VoiceOver before you start. this can give you audio clues with the volume control.

I did not do it on purpose, but it saved me, because I got the black screen.
I also had to re-insert the display cable for like 3 to 4 times until it worked for me.

(Note: Check if the display port cover is hinged on the battery side, otherwise there will be room for the connectors to come loose.)