MacBook Air A1466 screen works after connecting to external display?

Hello, I have a Macbook Air A1466 from my customer. The problem is that the display is not working. It is completely black.But when I connect to an external display the screen works again on the Macbook Air.

It shows image on external display as on the Macbook air display. And after I disconnect the cable from external display, the screen continuous working.

But after few weeks this problem appears again. Does anybody knows this problem and how can I fix it?

Thanks a lot!