Hi, you count your weeks of pregnancy as of the first day of your last period. Since heart is forming at week 5, it should be heard by intravaginal scan at week 6 which they call viability scan.
I suggest that you rerun a blood test in 48 hours to make sure your beta HCG level goes up, it should double every 48 hours with an healthy pregnancy. In the contrary if it goes down it's not a good sign.
Maybe you just miscalculated your calendar and your embryo didn't reach that stage of development yet.
Or your baby as Chromosomal Abnormalities and natural selection is done and an early miscarriage is on it's way. It happened to me last week and I was told that it's very common 60% of pregnancies.
I hope it's option one for you because even if we know all the facts, that it's better to happen in the first semester or before you hear the heartbeat, that the next pregnancy will give you a healthy baby bla, bla, bla, it's still hurts, you need to mourn that lost and all the projections you had made for your family that would add a new member to it's ranks.
I wish you blessings dear.