Hi Did you mean the backlight is on , but the internal screen is black? Have you checked your settings through the external monitor? You may want to try resetting the pram and smc .
After resetting the PRAM and SMC, I would try holding OPT at boot. This should bring up the drive selector however that is handled by EFI and not the OS. This would determine if it is hardware or software. My guess is the LCD cable.