So I was working as normal on a project of mine, when suddenly zbrush decides to only display a flat shader for everything. I don't believe I lost any of my textures or polypaint data, it just won't display anything but a flat shader on my tools. This is the first time I've encountered a problem like this. Any idea what's going on? I can't change the matcap at all. If I choose a different matcap nothing really happens. This is really frustrating.

can you post a screenshot of the zbrush window?

Check the Render palette to make sure it's not on flat. Chances are that's not the problem though if you can't see polypaint either.

Are you on R3?