Thanks for posting your query.
As you had your ultrasound
that showed fetal poles and heart activities suggesting viable/healthy pregnancy
but after few weeks your ultrasound show no fetal pole and yolk sac
without any sign of miscarriage
like vaginal bleeding/clots and pain, it suggests missed abortion
, the embryo dies without your knowledge, and does not deliver. It is possible to have missed abortion spontaneously, many factors can cause it like chromosomal problems,unhealthy lifestyle,maternal age,trauma or maternal diseases.
Repeat your ultrasound.
You should wait for few days if products are naturally expelled.
D&C may be required for retained products.
Talk to your doctor how to avoid miscarriages in subsequent pregnancies.
Hope it helps.