If your chlorine level got to zero, then soil bacteria that can get into the pool can grow and convert the CYA into ammonia. If this happened, then it might take an extraordinary amount of chlorine before you start registering FC. If this seems to be the case for you, then get an inexpensive ammonia test from a pet/fish/aquarium store. You would need to add an FC amount that is about 10x the amount of ammonia you measure.

Richard