Why is my screen not working after replacement?

So I have a cracked screen but it was still working it wasn't bothering me that much however I decided to replace it.

I bought a screen from Amazon and replaced it but the screen doesn't turn on. The phone is working because it vibrated when I plug the charger in or mute it.

I returned the screen and got them to send me a new one but that doesn't work either. I put the old one back but that doesn't work now.

I looked at the flex cable ports and then seem to be undamaged. What could the problem be?

Many thanks in advance.