It could be another week before your baby has a heartbeat, as long as there is a fetal pole you should be fine. I would repeat the ultrasound in one to two weeks to reassure myself.
At six weeks old your unborn baby is very very tiny and is only about the size of a pea, making the heartbeat difficult to find. There is also the possibility that you were inaccurate with estimating how long you are pregnant which can make the baby even smaller.
The baby is so small at six weeks that it is very likely that a doctor will not be able to detect a heartbeat. Most babies' heartbeats first become discernible between nine and twelve weeks. Your doctor will be able to use Doppler to listen for a heartbeat within the next month or so.
That is so scary, but the worst thing you can do is stress out. There is a higher chance that something is wrong since no heartbeat can be detected, but it may be that you are actually less then 6 weeks pregnant, so it can't be seen yet.