It will come back during her next molt. (Shedding of the exoskeleton).
Often the crayfish eats the molt as a source of calcium- therefore, you may never have seen the cast-off exoskeleton. Have you ever seen the animal looking more pale than usual, or blotchy? You may have just thought it was camoflage, the crayfish trying to blend into the surroundings. In fact, if you have seen this, it may have meant that the exoskeleton was softening and getting ready to come off. Do you have other fish or crayfish in the tank? They could easily have eaten the shed, too. Hope this helps.