Becube - welcome to TSF
Sounds like the backlight has gone out (display light). There is a slim cold cathode tube that lights the display. I would call dell and ask them about it. Additionally sometimes on Dell laptops there is a FN key (usually in blue) in the left hand corner of the keyboard, if so hold that key down and try to adjust the brightness/contrast of the screen by tapping the keys in the upper right of the keyboard that have blue symbol for contrast/brightness.