Dispaly light off at playing a Videofile but sound...

Question

During playing aVideo file or a game display light became off sound remains ok but program or application is running&continuing.This prolems occours even at call or taking an Image.At that time sceen saver remains ok.Atfter switching off& restat it will work again.This problem occurs maximum times in Video paying,Game,Applications use rare in Calls&Image taking.Even this problem when charging phone.How this DISPLAY OFF problem solve?give asolution.