Sac but no baby?

So my partners daughter is pregnant, as well as me. 
She got taken to A&E last night because she was having really bad cramps... 
She says she is 6 weeks, they rountinly done an ultrasound and saw the sac but no baby? 
She has to go back next week for further tests? 
Can someone give us an insight as to what's happening? I feel awful as my pregnancy is going really good and I have a healthy baby but I'll feel so awful and guilty if something is going wrong with hers?
Thanks X