theres a strong possibility that he/she just died and the tank ate the corpse... you may want to make sure all is well before adding another!

maybe a water change, run some carbon and a solid round of testing??? There are lots of reasons that a change like that could "shock" a tank. Maybe the coral (if any) got stressed enough to toxify the water, maybe the fish got stressed enough to loss the slime coat and was eaten by the anemone (if any)... etc...