Possible dead screen or inverter/backlight. Try looking very closely to see if you can see anything at all on the screen.
If you have an external monitor connection eg VGA HDMI try using it connected to a display. You might have to hit a couple of keys on your keyboard to swap to that output, like Fn+F12 depending on your laptop.
Someone might have accidentally turned down the brightness via the keyboard, or switched the display output to your external source (as above)
A make and model would be nice!