Help! 7week 3 day ultrasound, no heartbeat.

I had my first ultrasound today at 7 week 3 days and baby measured 6 week 5 days and no heartbeat. Doctor said the sac looked great but was not optimistic about this being a viable pregnancy. I go back next Monday for another ultrasound. Anyone have this happen and have a positive outcome? I want to stay positive but I feel crushed.