Problem with textures in portal 2?

When I play the game, certain things in-game such as buttons and vines show up as a purple and black checker pattern. I've looked up how to fix this, and it says to verify the game cache. I tried to do that but it wasn't doing anything. It just said 0% complete, and I left it on for a good amount of time. The only thing running at the time was steam. Is there any way to fix either of these problems?