screen blacked out . no brightness

I replaced my macbook airs screen from a third party 4 months ago .

A week ago my macbooks screen started flickering and today it completely got shut , after i noticed that the screen or picture is still showing but its very dark and also its working in my external monitor completely fine .

It may be a problem of backlit , but i am not sure

so if anyone can tell me what could be the problem & how to fix it .

any help much appreciated.

thanks