The screen on My wife's HP 4540S has stopped working.

The laptop starts and runs OK, it will drive an external screen.

The laptop screen is not entirely dead as it changes from a dark screen (but one that shows the glow you see when the screen is blank when a computer is on) to completely black and back again like happens during the boot up process. There is never any graphics displayed.

I tried the trouble shooting suggested on the HP website but no luck.

Is it likely to the the screen, the cable or the GPU. I figure that since an external screen works OK, the GPU is probably OK.

Any help appreciated.

Thanks