2010 MacBook Air Black Horizontal Line Across the Screen I have a 2010 MacBook Air with a black horizontal line across the screen. as shown in the photo below.   I don't know when it started; I just remember that I opened the lid today and saw it with the stripe. 

As far as I know, it's not a GPU issue because connecting it to an external display does not show the problem. I've tried plugging and unplugging the display cable (no results), so I'm not sure if there's anything else I can do before I consider buying an used screen.
It's most likely your LCD screen.   

Your MBA has a single integrated GPU that's used for both your internal screen and to drive an external monitor.  The fact that it's not showing up on the external monitor narrows it down to the LCD screen.

Additionally, if this is happening prior to boot up of macOS, it's happening outside of the operating system so it's definitely a hardware issue.

It's possible to DIY replace the screen (Apple PN# 661-5732), but be aware that iFixit.com rates it as "difficult"