Confused mama! Gestational Sac no embryo

Hello. Im hoping someone can help me. This has caused me a lot of confusion and anxiety.

I was bleeding for about three weeks (on birth control). My dr is out of town so I had to go to an urgent care who suspected I miscarried but did not have any ultrasound machines. I was able to get an ultrasound somewhere else and the tech determined a gestational sac measuring at 5.5 weeks but no embryo. I took a urine pregnancy test which was also negative. Has this happened to anyone else? Is the miscarriage complete? Will the gestational sac disappear or pass now? Could this mean something else like a cyst?