iPhone 6 will not respond after replacing screen and battery.

Hi guys and gals!

So I've received an iPhone 6 from a friend who was for sure, 100% positive that the phone would never work again, but I asked to just take a look at it to maybe see if I could possibly bring it back to life.

Challenge accepted!

So story of this phone to say the least it has been through h*** and its most recent damage it has been through is that the screen was rolled over with a skateboard; so he ripped it's original screen off from the LCD and digitizer cables and just left the connections attached underneath the connection plate.

Well long story short, the phone hasn't been turned off since that point in time...(8 months ago...) so I replaced the battery and I just momentarily plugged in only the screen itself to see if it would even turn back on and it did!!

So I put a new home button on, with a new LCD plate, and a front camera and everything else, and now the phone doesn't respond. The only thing it will do is when I plug it in, it vibrates continuously but nothing comes to show on the screen. I've changed the battery once again with another new battery, and I've tried multiple screens and it continues to do the same thing.

Help will be greatly appreciated!