Why is the screen not showing?

I recently cracked my iPhone 4S screen and bought a repair kit to repair the display case. I installed the replacement screen properly (being careful to install the digitizer and display cables properly). After putting it all back together and plugging in the phone to an AC power outlet, the screen does not light up and display the icons like before. I can touch the blank screen and hear the clicking noises and also somehow get to my dialing application and hear the touch tones when you press each number. It is receiving power properly, but I can't view the actual screen and know whether or not it is turned on. Does anyone know which part I'd have to replace to fix the screen problem? Also, in toying around with the phone in replacing the old screen with the new replacement screen back and forth (I originally thought that it was the screen problem), I stripped some of the screws clean with the tiny screwdrivers, so I can't get back or separate some of the parts from others any longer. Does anyone know of a proper technique to remove these stripped screws? I feel as though I may have a complete mess on my hands. Any help is appreciated!