Just Sac at 7 weeks and no baby

Please I need some positive hopes!
 I'm 7 weeks and 3 days according to my last LMP. Went in for an ultrasound yesterday and all they could see was a yolk sac and no baby 😞. My ultrasound measured I'm 6 weeks and my gestation as 5 weeks. I'm really worried. Initially my hcg was doubling everything looked fine. Now my doctor is really concerned. She said I might have a miscarriage. I'm totally devastated. I'm going next week again for an ultrasound. But I lost hopes after yesterday's update. Can anyone help me with some insights??? Did this happen to anyone before and did you see the baby and the heartbeat later? Please help