No backlight, black screen, external monitor works ok

I use my laptop mostly with external display via displayport, but laptop always in open state(use internal monitor also).

No water or drop damage -- on half of working day internal monitor became black but external continue working. I try reset/boot macbook with keyboards shortcuts but with no success, so probably hardware problem. I use flashlight to check display and i can see that display working.

In most cases to which part of display(or whole display) this issue could be related from your experience, and from which point i need to start ?

UPDATED: i can see interface if i use flashlight.

Thank you!