I had that happen shortly before the return spring for the paddle broke. Basically what's happening is the escapement on the paddle isn't disengaging properly, which locks out the thumb lever. The inside of an ultra shift lever is really simple, it's basically a friction shifter spool on an axle with two ratcheting adjusters (the thumb lever and paddle) that move one way and don't engage when it moves the other direction. If one of them locks out or that direction is otherwise immobilized, then the shifter locks because the ratchet is jammed. Grit, lube failure, spring slip or breakage, overtight pivots, an overly tightened main screw on the shifter axle itself, an undertightened main screw, and even a fraying cable head could cause this symptom.