How to fix a display that is black and white lines?

The touchscreen itself works. I can get to games since I memorized where they are on the gamepad screen. The screen will light up, but it only shows black or black and white lines. So I can't do anything in the system that displays on the gamepad itself since I cannot see what is going on.

I bought the wii u and gamepad refurbished about a year ago. The gamepad worked for about two weeks before the lines started showing up. It got progressively worse until there was no display at all pretty quickly. I didn't really care since I can get to Zelda and Netflix without the display, but I changed my Nintendo ID password the other day and now I need to enter it on the wii u which I'm finding impossible without the display.