Troubleshooting no image or backlight

Hi there!

Recently changed the screen of my iphone 6, I tested the screen without closing the phone and it worked perfectly. After completely mounting it it didn't turn on at all.

Reconnected the flex cables just in case they were not connected properly, nothing.

Reconnected another known good screen and still got no image or backlight.

The phone turns on and iTunes recognizes it tho.

I am pretty sure it is not longscrew damage since the screws were in their correct position. What are the steps to troubleshoot this issue?

Thanks!