The fact that a heartbeat was seen before is good, and if you are talking about the portable scanners, no they are not as sensitive, especially for such a tiny foetus. I had to wait overnight after one of those scanners showed my DD2's heart had stopped, and she was nearly full-term, so big enough for them to know, but they can't confirm with them.
If the heart has definitely stopped, then unfortunately it is game over. However, they have just said they can't see it, so there may be hope. The foetus itself wouldn't grow, if the heart had stopped, but there are so many variables - if the heart had just stopped, or if it was the gestational sac, not the actual foetus that they were measuring.
Please don't get too despondent. If the baby was bigger, then it would be almost certainly bad news, but those portable scanners are not brilliant, so try to hang-on it there till Wednesday. I know how hard it must be.
(((hugs))) xx