My friends belts on his jk were stuck after being soaked by mud. Inside there's this little ball with a spring. If the belt pulls out fast (like an accident) the ball is thrown out, locking the belt. Dirt and mud can get in there and cause the ball to get stuck in the locked position. Sometimes you can soak it with WD-40 and release it, but on both of my friends assemblies we had to open them up and remove the mud caked inside on the ball.
It doesn't sound like this is your problem because they worked fine before, but just in case you need to take it apart, check that ball. Best of luck!