My Macbook Pro, when awaken from Sleep Mode does not activate the screen. The computer comes on alright but the screen remains dark (off) though the keyboard is lit and I can hear activity. This happens often, whenever I close the computer and then reopen it. This has been happening for while. And it's not the brightness, cuz I turn this to the highest and it remains dark. Then I have to restart it inappropriately. This is very bad and unproductive. Please help!