Hi Eileen. I'm so sorry you have had this fright and I hope it all turns out fine.
Were you exactly 6 weeks at your first scan? Mine was at 6+3 (in fact, once my dates were later worked out, it would have been 7 weeks) and they only did it that early because I went on holiday the next day. Normally they like you to be at least 7 weeks to avoid this kind of thing. The pole doesn't start growing until around 6 weeks and it's also hard for them to find a heartbeat that early. Also, if your embie implanted a day or 2 late, you'd only have been less than 6 weeks.
I won't tell you not to worry, but I hope I have given you some hope that the next scan will bring you better news.