My granddaughter had an abnormal hemoglobin test result, has to be retested right away, how serious could this be?

Depends: Is it a low hemoglobin level or an abnormal form of hemoglobin? A low hemoglobin level is generally associated with anemia, and can usually be treated (for example with iron supplements if it's due to iron deficiency.) an abnormal form of hemoglobin is different. It can be detected on the newborn screen or other specialized tests, and can be associated with sickle cell anemia or thalassemia.