iPhone 6 won't turn on after replacing battery and screen

Hi guys,

I really need your help. I have got an iPhone 6 (don’t know it’s history) other than it just didn’t turn on and didn’t work by it’s previous owner. So I’ve replaced both the battery and the screen but it’s still the same. When I plug it into iTunes, the phone buzzes three times and the black screen turns a very pale white (or lighter black if you like) and it looks like it’s about to turn on but nothing happens. Also, iTunes won’t recognise the phone. When I try to turn the phone on using the power button (without any cables), nothing happens on the screen. When I plug it into the wall charger, again nothing happens. Don’t know what else to do. I’ve also tried holding down the power and home buttons together but nothing. Can’t even see the Apple logo. Has anyone else had this experience? What else can I try? What could be wrong with it? Your help is kindly appreciated.

Cheers