Ferritin is a measure of CNS (central nervous system) stored iron. It can be very low, particularly with certain diseases such as restless legs syndrome. In cases it has been in the single digits, but I have never heard of it at zero.
Ferritin is found in many tissues and is closely related to the iron stored in your body. Very low serum ferritin levels are seen mostly in iron deficiency anemias. If your blood count was otherwise normal this may just be a lab error.