First try the torch trick to see if it is a LCD backlight (or lid switch) problem.
Turn on the laptop and when you think that it should have booted all the way to the Windows desktop (check HDD activity light has settled down), shine a torch at an angle close to the screen and check if you can detect the Windows desktop image. (you may have to peer closely to see it, as it will be faint)
If you can detect the desktop image then you either have a faulty LCD screen backlight power circuit, a faulty lid switch or a faulty LCD screen.
If you cannot detect the desktop image then connect an external monitor to the laptop and see if it displays OK when the laptop is turned on.
If it doesn't then you either have a faulty GPU or a faulty motherboard, (usually the same thing if GPU is mounted on the board).
If it does then you either have a faulty video flex cable, faulty video flex cable connection at either end (motherboard or screen) or a faulty LCD screen.
What is the model number of your laptop?