My Mac book is working but the screen is super Dark

When I booted up my Mac book air 13 inch I could hear the start up chime but nothing was appearing on the screen. So I rebooted serval times until I finally made up the faint shape of the Apple boot up logo on the screen. I experimented for a while a realised that if i was to put a torch up to the Apple logo on the back of my Mac I could see the part of the screen in front of the logo. I linked my Mac up to a external monitor and it worked fine. every thing on the screen was working fine except I could not see what was on the screen.

Any Solutions on how to fix this?