There is a lock nut inside the knob whose purpose is to keep the knob from spinning off completely. Sometimes it works it way into the other nut and makes it impossible to to rotate the knob. Pop the top off of the black knob with a small screwdriver (there is a small slot to insert the screwdriver). Once the top is off you'll see the lock nut -- just loosen it 2-3 turns with a socket wrench, snap the knob cap back on and all will be normal again.
I need to try to remember this so when it happens I know what to do.