At 6 weeks I had an ultrasound done and we did not see the heartbeat, it was just too early. At 9 weeks there was a nice strong heartbeat! I've had a missed miscarriage and it is very possible to have no heartbeat and no bleeding or in fact no sign of a miscarriage, that's why it's called a missed miscarriage. With how early you are I'd say relax and wait for the next scan and see what is really going on.
It could be that it is too early in which case next week should give you more information and you may be able to see a heartbeat. However in early pregnancy it is not uncommon to have a missed miscarriage in which the baby stops developing but your body doesnt actually go throuh the process of a miscarriage. I had an ultrasound at 6w2d in which I saw a heartbeat went back 3 weeks later and baby was only measuring 6w4d and there was no more heartbeat. I ended up having to get a d&c. Im so sorry you are going through this. Hopefully your ultrasound was just too early and there is a heartbeat on your next one.
A heartbeat can't be seen until closer to 7-8 weeks, so if you happen to have been only 6 weeks, it was too early to see anything further. You might have ovulated a bit later and since she saw the sac, maybe you need another week or so to see if it continued to develop. Don't give up hope.....I am glad she made another appointment for you. A few days can make a HUGE difference. Good luck!
If you are only 6 weeks pregnant then it could be too early to see it. You might be even further behind than that, since they had to do an internal scan to even see the baby. Kids grow at different rates - be patient (I know it is hard) and see how the next scan goes. It is probably just too early to see it.