I believe it has something to do with the metal on the back of the LCD making contact with the components underneath (so not the touchscreen as such) there is a black plastic liner around the screen that prevents this from happening. Just check it is still in place.
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see this [http://www.ifixit.com/Answers/View/47554/DS+Lite+won%27t+turn+back+on+after+replacing+bottom+screen.|answer]
I believe it has something to do with the metal on the back of the LCD making contact with the components underneath (so not the touchscreen as such) there is a black plastic liner around the screen that prevents this from happening. Just check it is still in place.