Are you talking about the horizontal line through the center of the screen? Let's see if I can remember. That has to do with the line that appears as the GB is turned off. The display turns on the middle row at max intensity for a few seconds for some reason, which shortens the useful lifespans of the dots in that row. Nintendo figured out how to prevent it in software, so all games produced after a certain date prevent that line from appearing, but older games still abuse the screen. I don't think you can do anything to restore it.