Talk to Gynecologist on Strip Pregnancy Positive But Nothing In Ultrasound

We have precived pregnancy after 2 years of marriage with the help of oviolin 50 mg. Now, we have 6 week pregnancy. Urin tests from lab shows positive (faint dark line ), in ultrasound doctor says I can't see any baby. What is possible casue for it, what we should do next ? Someone suggested that maybe pregnancy happen in flopian tubes ? how can we be sure for it to avoid any complications. Any blood tests or other tests for it ?