6 weeks pregnant with normal HcG but no baby on the ultrasound?

I went into the ER yesterday with a panic attack, and because i had not had an eval for my pregnancy yet, they did that too. Took my HcG reading and it is right where it should be, if not a little higher then they would normally expect, but when they did a vaginal ultrasound they couldn't find the baby. No sack or anything.. I'm pretty scared.. Advice?