I was about to climb a hill, but was still on the flat when it happened. As I shifted from big to little chain ring (compact double), my rear wheel suddenly locked. I was in the middle of my cassette. I got off, and I could pedal backward, but not forward at all. The chain was not off, and I couldn't figure out where it was jammed. I released the rear wheel quick release, and it unjammed.
Any ideas what that could have been?

