I had my ultrasound today and my ob said the fetus is too small to measure. Now I'm worried that the fetus has no heart beat. I'm estimated 5/6 weeks?

Too early. It is too early to worry. A fetus can not usually be seen until at least 6 weeks. Most likely the doctor scheduled you to return next week for a repeat sonogram. At that time you should be able to see and determine if all is well.
Normal. The heartbeat doesnt occur until 12 weeks gestation. So if you are estimated at 5 or six weeks, that is within normals. A safe and healthy pregnancy wish to you.