iPhone 4 Not Working at all after New Screen Replacement

Hi,

I recently bought a fully working iphone 4 with a cracked screen and then very carefully replaced the broken screen with a brand new one, put the phone back together and it now does not work at all.

I've tried charging it, nothing.

I've tried holding the home and power buttons together for a while, nothing.

I've even tried opening it back up to check all the ribbon cable etc are connected properly and they are, they're all as they should be! The phone just wont turn on at all!

The only slight bit of life is whilst the phone was being charged for the first time post screen replacement the black part of the motherboard in the middle got quite hot and stayed quite hot for a while even after I'd un plugged it from the charger. even before this charging it still didn't turn on at all and before I'd done any of the dismantling the phone battery was about 75%.

However I'm charging it now and the motherboard doesn't seem to be getting hot, this is all a complete mystery to me so I'm calling on the experts!

I really could use some help here! Thanks, Matt.