Connect an external monitor to the laptop and check if it shows a display OK.
If there is a desktop display on the external monitor
Use a torch and shine it at an angle close the the laptop's LCD screen to check if you can see a display on the LCD screen when the laptop is on. It will be very faint so try it in a darkened room to make it easier.
If you can detect a display then you have a backlight problem. This could be caused by either a faulty backlight power supply, a faulty backlight power cable, a faulty lid switch or a faulty LCD screen
If there is no display then you either have a faulty video cable, faulty video cable connection (at either end - motherboard or screen) or a faulty LCD screen.
If there is no desktop display on the external monitor then you either have a faulty GPU or a faulty motherboard, (which is usually the same thing as the GPU is permanently mounted onto the motherboard)