Macbook Pro 2014 doesn't recognize internal display
Solution 1:
The fact that your internal display failed, but not your external ones says that the GPU is working. It suggests that your display is bad, however, it's doesn't rule out the logic board as it could be the video mux chip.
The only way to positively diagnose this issue is to try a known working display on your Mac or connect your display to a known working logic board.
Since I doubt you have spare MacBook computers laying around, it's best that you take it to a repair shop for diagnosis.