first ascertain if it's just a faulty screen by plugging in an external monitor.
If external monitor works okay, the laptop screen's backlight has probably blown and needs replacing.
If it is just a blown backlight, you can usually see a faint image on the screen by looking very closely at it whilst shining a torch on it (with the laptop turned on of course).
If you see a faint image then it's definitely a blown backlight.