iPhone 4 screen physically unharmed, but has scrolling screen. Tried tapping the corner, will not fix.

Last night my iPhone 4 slid out of my back pocket at the movie theater. It was turned off completely at the time, and when I turned it on again, it had a scrolling screen. My friend had the same thing happen to her iPad so she suggested tapping the corner of the screen. It worked the first few times, but throughout the night it happened more and more frequently and with less success with the tapping. Now it won't fix at all and I've tapped it on a wall, firmly, on my knee, set it down on a desk in hopes to fix the problem, but it still won't work. I'm not sure what the base of the problem is, or how to permanently fix it. Any help would be greatly appreciated. The screen looks like an old worn out vhs tape, faded, with black lines running through.