Why would there be no display?! blank screen ...

I just replaced my third iphone 6 plus screen using the excellent step-by-tep guuide found on this site. I did a display test prior to securing the "front display cable assembly plate" ( the small one in the top right of the phone) the screen came on, it was beautiful. i then powered off the phone, propped up screen and began to screw in plate. long story short, the display is blank now. The phone will not power up or display a thing!!

I am at a loss. nothing was done between powering down following the display test and the final screw being put in the plate .....