Hi,
I am so sorry to hear you are going through this. It is unlikely that the fetus is just growing slowly, or that there was a mistake on the scan... there is a possibility that you had your dates wrong, and that the pregnancy is earlier on than you had thought.
If your dates are accurate, it is likely that the pregnany has stopped progressing. If that is the case, I am sorry about that. With a missed miscarriage, you will likely be given the option to wait until your body passes the products of conception on its own, or to have a D&C.
Miscarriage is very common, and many women have a miscarriage before they even realized they were pregnant, so don't let it discourage you from trying again. Most women who miscarry go on to have normal pregnancies, although I know that does little at this time to make the pain of this possible loss better.
Best of luck,
JMK MD