my screen works but it's like it was always off

Hi, let me explain a little bit more, my screen works perfectly i can take screenshoots, call, writte message, and all but only using siri and good luck because the screen it's always off it doesnt turns on but it works perfectly, my cousin opened it up because my guaranty it's over and he put my screen on his phone and it totally worked..... wat can i do?

HELP ME PLEASE