No backlit keyboard???

I have a new macbook Air (1 week and 1 day old). Last time there was a backlit keyboard. I think i pressed something in system prefrences cuz i was exploring my new macbook air . Can you tell me how to switch it back? Thanks. When I press the F6 key which is to incease the brightness of the backlit keyboard, it will not increase. instead, there will be circle with a line across the space within the circle. It just does not light up the keyboard. Thanks again.