Phone [Q] LG Optimus G - no screen response

I have this LG Optimus G for about two months now. i recently dropped it and it cracked the back of the phone.
everything worked just fine afterwards. but now the upper half of the screen won't work. it shows what it should show though when i touch the screen it doesn't respond. the lower half of the screen works good though.

WHAT COULD BE WRONG?

p.s.
sorry for my english. i'm not a native speaker.