Why wont my touch screen work?

I have an iphone 4 and my touch screen doesn't work because i had broke a connector off of the screen. That being said i got a new screen from my friend and put it all together like a new phone. Now the screen still doesn't work just like it was before. Could i have damaged the motherboard so it wont work ever again? Also i tried resetting it by the hold button and home button and still doesn't work. Also i got another motherboard from a friend and put that in and the screen would blink once when i pushed the home button or hold button whats wrong? EVERYTHING ELSE WORKS I CAN SEE WHEN PEOPLE TEXT OR CALL ME AND I CANT RESPOND!