Black screen, but everything else works

Last week, my mac's screen suddenly became black. However, everything else works (including the backlight), as I can use it by connecting an external monitor. I tried everything, the control+option+p+r thing, resetting the SMC multiple times, disk utility, !&&*, I even did a clean install of the system and the screen is still black. The mac is less than a year old and I never misused it, I guess someone bumped into it while I was not home.

I want to open it up and see if there's a loose cable somewhere, but I'm not sure what to look for... What are the things I should try before coming to the conclusion that something is indeed broken?

Thanks!