What does it mean if the ultrasound did not see anything in the sac?

Blighted ovum. That would depend on how advanced the pregnancy is. By 5 weeks there should be at least a yolk sac present in the gestational sac in the uterus. By six weeks a fetal pole should be seen and 85 to 90% of the time a fetal heart beat is noted by six weeks. If there is no fetal pole or yolk sac present by six weeks, it is unlikely that the pregnancy is normal. That is referred to as a blighted ovum.
Blighted ovum. A pregnancy with a sac but no visible fetus is often called a, "blighted ovum" if the sac is over 18 mm in diameter or if the sac is over 13 mm and no yolk sac is seen. It is felt that an embryo never formed and only the placental components of the pregnancy developed. Since the placental tissues produce hcg, the pregnancy test will still be positive.