First turn off the laptop and connect an external monitor. Turn on the external monitor and then turn on the laptop. If you can see on the external monitor, then the problem lies in the attached display. If that is the case you would need to check the ribbon cable that connects the display and make sure it isn't damage, loose, bent, torn, pinched, etc. If it isn't, then the display itself may be the problem.
If you can't see on the external display either, then your graphics card/GPU (and possibly your motherboard) will need replacing.