Hi,
Have you tried adjusting the brightness of your display? On your keyboard depending what brand it is you, can just press shortcut keys to adjust the level of the brightness. If these doesn't work do try connecting an external monitor and see if the problem will persist because if it's not that means its the built in display that has the problem. Next is to uninstall/reinstall the graphics card driver from Device Manager.
- In Device Manager uninstall the graphics card driver.
- Reboot your laptop and it should reinstall the driver automatically.
- After the reboot see if the same problem will persist because if it does that means it requires a replacement display already before replacing it make sure your reseat the ribbon cable...