LCD Replace does not stay on.

Hello!

I am having difficulties with a repair.

I have an iPhone 6s that needed a screen replace. After doing the repair the screen worked just fine and it was ready to give it back. The problem appear the next day when I tried to turn on the screen and it didn't.

I hard reset the phone (home + power) and the iPhone came on with the apple logo really obscure. It looked as if the light were not working properly and were off. After that the screen came on and I was able to use the phone but when I locked the screen and tried to unlock it, the screen did not come back on.

At first, I thought it was the screen but when I replaced it with the cracked screen it had the issue was still happening.

Does anyone has an idea what could of happened.