Cannot select anything all of a sudden. WTF?

So.... I have no idea what I did, but all of a sudden I cannot select anything in a sketch. Normally, when I hover over something, it turns orange. Now, nothing happens.

I discovered this when I attempted to select a midpoint of a line, and hovering over where the midpoint should be resulted in nothing happening.

How can I return to the usual behavior of being able to select sketch entities?