Gray lines through the middle of the iPad screen.

This happened 3 times actually, I dropped it by accident and when I pick it up, colored lines appear through the screen. I took it to the apple store and this guy at the genius bar bent the corners of the screen and fixed it.

The 2nd time, the same thing happened but then the lines were gray instead of colored, I got it fixed at the apple store but they warned me that I have to be extra careful with it or it may break again.

And the 3rd time it broke, I bent the corners of the screen and I banged it into the wall lightly and it seemed to work again, when I left it overnight however, the same problem exists.

How do I fix this and what is happening?

EDIT: By bending the corners of the iPad again, the color was changing and suddenly the screen was back to normal. But I'm scared it might happen again.