Why does my Iphone 4 not activate upon touch?

I switched my white iphone 4 screen onto my black iphone 4 because it was cracked, after finishing up and closing it i accidentally tore the little thing attatched to the screen. When I turned on my iphone it doesn't recognize my touch, what do i need to do to get it working again? A new screen? What is an LCD, Digitizer, and glass for? Please explain. Much help needed.

You need a new digitizer.
if you in PA email me i have a few iphones for sale or trade

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