no visibility of display screen

I have HTC One M9 Plus phone..the phone was great, but recently the phone was shutdown black screen,i thought it was totally dead but when i switch the power on, ican hear the sounds loading the OS but i cant see any display on the screen clearly, i mean it was open but it seems no light,i can just see the display when i flip the phone towards the lights on or sun... what should be the possible trouble of the phone?..please help me