iPhone 4S won't turn on after Battery removal and reinstallation

Hello,

My power and volume buttons were getting loose so I had to open my phone up and tighten those screws, I had to remove the battery and the shield to get access to it. After putting it back together it wont turn on! I did double check every flex cable/connector, and checked if the battery was properly fitted so I know that isn't the problem.

However the screw holder that would screw in to the top part of the plate that covers the battery & antenna shield has fallen out, and I am wondering if that could be the possible cause of my phone not booting. If not, what could be?