last week ultrasound with embryo. after a week, no embryo, only sac. 5 weeks

My ultrasound result last week showed an embryo in the sac.. With heartbeat at 77bpm. That was only 4 weeks. Today i had another ultrasound by a different sonologist and clinic, now at 5 weeks, but he saw only a sac, and no embryo. Im so lost.... :-( can anybody shed light to this? Thank u :-(