Mid 2012 Macbook Air Black Screen
It sounds like your backlight has broken.  The LCD screen needs to be lit from behind before the screen contents are more than vaguely visible.  If you boot the machine again, and shine a bright torch from the front, you should be able to see your usual screen layout for login etc, but obviously everything is going to be very very dull.  If this is the case, then you can get that fixed with relative ease at various repair centres or through a slew of online guides.

Sometimes on older hardware it's not the backlight that's gone, but instead the tiny starter/transformer that is required to kick a backlight into life, although this type of method isn't used on modern LED backlit screens like your Macbook