No heart beat at 7 weeks - help!!

I am really hoping there are some stories to put my mind at ease but I do fear the worse. I had a scan two weeks ago where they saw yolk and sac - nothing else. Then today i had another which showed a 5.5mm baby but no heart beat. By my calculations, I'm 7 weeks 2 days preg, but obviously could be wrong - say by a week MAX. Should I give up hope? I now have to wait ten days for a second scan. Please help if you've had ay experience of this.