Shower faucet handle freely spins - doesn't shut off water

hi there, We have a stacked Kohler shower faucet. The faucet that controls diverting the water or turning it off, the handle goes round and round but has no effect. if I take the handle off, i can reach in with needle nose pliers and turn the splined stem and the water shuts off. What gave way that needs replacing? Stumped.