Two possibilities - if you have switched the display to another device - this would use the function key with what looks like two display screens - with your pc switched on try holding down the fn key and pressing the key with the two display screens on it. Try this a few times allowing a few seconds between each attempt. Another (less likely) possibility is that you have turned the screen brightness right down. You should see a function key with what looks like a sun on it together with a plus sign Try holding down the fn key together with the key with the plus sign on it.