empty sac at 6 weeks..

I went for my 2nd ultrasound today. During my first ultrasound last week we discovered that I was pregnant with twins. And today ultrasound has showed only 1 sac, which got bigger but was empty. Now we wait until friday when we have another ultrasound to check if there is a baby.. I should be 6 weeks today. Did any of you ladies had the same situation and the pregnancy progressed normaly wih healthy baby?