Screen black afer opening

Hi

I wonder if anyone can help with this issue.

I recently dropped my iPhone 6 + and cracked the screen. After looking at a few youtube video's I bravely and stupidly decided to carry out the screen repair myself. I managed to successfully replace the glass and put the phone back together. I opened the phone again as the ear piece sounded faint when making and receiving calls assuming it was not seated correct. Once the phone was put back together the display would not work.

i re-seated the display ribbons several times but the screen would not turn on again. I also tried a completely new screen and again the display remained black. The phone definitely turns on as its recognised when connecting to iTunes and siri also works.

i also disconnect the battery, re-seated the display ribbons again and connected the battery again with no success.

If its not the display, can anyone advise what it could be

Mitesh