The screen is made from 2 very thin sheets of laminated glass with liquid crystal (LCD) sandwiched in between.
In front of the screen there is a plastic lens (the bit you can touch) which will flex in, but if you push it in too hard (which you have done) the glass part of the screen will break and the LCD will leak out to form a dark mark on the screen.
I've never known the LCD to leak out of the phone, but once the screen is cracked none of it will work.
Adam
July 2009