I had an abdominal ultrasound which showed the pregnancy at about 5 weeks, but no fetus was seen. What should I do next?
Don't jump to any conclusions. An abdominal ultrasound can easily miss a normal pregnancy at this early stage. Your next step should be to have a vaginal probe ultrasound. It is the single best view of early pregnancy you can get. If the vaginal probe ultrasound is suspicious, then I would recommend you check your hCG levels, to see if they double in ~48 hours.
Make sure to talk to your health care provider about your concerns.