Lost twin?

A couple of months ago I joined this group because I was told I was expecting twins. A couple of weeks later at an OB appointment only one baby was found. I just recently got copies of my first US pictures when twins were found, and it includes notes from the US technician. In the notes it says there was only one gestational sac and one yolk sac, so I’m wondering could they have known one baby would not make it or would this not have affected the babies?????