Lines on handheld game screen

[GameHoarder]

I wanted to know what causes a handheld game screen to have those lines across the screen. And can the screens be fixed or cleaned?

The system that I'm wondering about at this time is the Pocket Pixter. The lines are vertical just as on a original GameBoy with a defective screen.

If there is information on these problems already in the Handheld Museum, where might I find it? I tried the search option for this forum but wasn't able to find what I was looking for.