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Assuming the screen itself works but it just won't respond to touch, it's not the screen that needs replacing.
It comes in two pieces, and the part you'd need replaced is called the digitizer (it's the glass on top of the screen with the black surround)
I've replaced this part three times on a couple of different 3Gs and it's probably the easiest thing I've done in terms of iPhone repair, costs about £15 for the part and takes a couple of hours since you need to be careful not to break a cable.

so as josh says it's a broken "touch screen" it's easy to replace then?
how hard are they to unlock too?
sorry for so many questions but this is new to me