Have you ever felt that you reached your mental limit?

Long story short, have been back to school for a little while now. As intelligent as I believe I am, I had one of those moments where I was staring at something for literally a day, all day long, and could not make heads or tails of it. It felt like I reached my minds processing capacity, and it had noting left to give. There are worse feelings in the world, but this was very annoying. Especially with a full semester of classes ahead of me.