broken key or a broken crank. if the woodruf key just sheared off, thats an easy fix.

bbuuttt, more than likely the pulley was loose and it walked back and forth over time, wearing a chunk out of the crank. if that is the case you will have to pull the motor and fix the crank or replace the crank depending on the damage. pull the pulley and inspect.

did you recently have a timing belt or any work done to that side of the engine?