Ultrasound at 7 weeks

So I had my first u/s yesterday at what, according my the calendar, should be my 7th week of pregnancy. The tech found no heart beat. I was left with no answers and she just said your doctor should call you tomorrow to discuss the results. Sure enough today I get a call from my dr's office, they said the doctor wanted to discuss my u/s results, nurse said there was no heart beat detected and dr wanted to get another one on wed. I'm losing my mind here and just wish they would tell me what's going on. Has anyone ever had an u/s with no heart beat that was later on detected? Please help!!