|When you tried another laptop screen did you also remember to plug in the feed for the backlight?|
Assuming you did so, and assuming it was a good screen, then it seems logical the fault is between the display output socket on the motherboard and the screen - this being the ribbon cable.
However, I've not known of a ribbon cable failing, other than being damaged when attempting a repair.
It's certainly not the graphics output side of things, or else an external monitor wouldn't work - although..., is there an Fn-key function that toggles between the inbuilt and external monitor? It should auto-switch, but maybe worth a look.