They should keep taking your hCG levels to see how fast they are rising, and do another ultrasound soon. If your hCG levels are slow-rising, it might indicate an ectopic pregnancy if they truly cannot see an embryo in the uterus. Either that or as Nichole said, it might be that you aren't really at 8 weeks. Sometimes that happens if someone has a chemical pregnancy immediately followed by another one without a period, they think they are 8 weeks along but are only 4.
Either you aren't as far along as you think or you have a blighted ovum. This is where there is just an empty sac present in the uterus, the fetal pole never forums meaning there is no baby. Your hcg levels will continue to rise since your body still believes its pregnant. What has your doctor said?
I think that means it could be too soon or a chemical pregnancy. I've read that. Hopefully someone with more knowledge sees this. Do they have you coming back to recheck??