Why is there a white line on the bottom of my screen?

I have this problem when this white line randomly shows up on my screen. I have an upon 4th Generation. It went on and of for a week, but then it got worse. It didn't go away and it made the screen lighter. I can barely read things on my screen and I can't see the bottom of the screen.How to I fix this and what its the problem? How did it get there?!?