I had no fetal pole, and baby stopped developing after 6 weeks?

The way my OB described it to me was that I had a sack with a yoke (I think), and no fetal pole. Does this mean at one time there was a baby, or did nothing but the sack develop? And why does this typically happen? He gave me a really great 30 minute science talk, but my brain was not processing due to the shock. I found out Sunday in the ER and had my followup with the OB today. Any kind of explanation would be helpful. Even though I saw nothing on the ultrasound today but the sack, I am super emotional :(