hi and thanks for your question,
missing to detect heart beat by a sonogropher with a pregnancy
greater than 12 weeks is very rare and unlikely. I would suggest you get another one for certainty. This should be done fast because, in case the fetus is dead, staying for a very long time could predispose you to other complications like disseminated intravascular coagulation
Doing another ultrasound
and rapidly if in doubt could help.
Thanks and best regards,