The problem I see is the small monitor is blocking the best parts of the image on the big monitor!
But I digress.
The fact the display is good on the external monitor indicates the notebook's graphics solution is still working fine. And the fact the notebook monitor is lit up suggests the notebook's backlighting is fine too. So the problem is either with the notebook's monitor screen itself, or maybe the connection to it (like a loose cable).
If you press the applicable Fn key combination to cycle through notebook only, external monitor only, both, then back to notebook, does the notebook screen display consistently fail to display properly?
If me, I would disassemble the notebook (or take it to a pro) to make sure the cable the notebook screen did not come loose or get pinched.
