vanishing twin syndrome?

I’m 10 weeks today.. baby #2 has always measured smaller by a week. They thought I was going to lose her but then they finally saw a good heart beat for both babies at 6 weeks.. I’ve been going once a week now for ultrasounds and everything has looked good until today… they couldn’t find a heart beat and the baby measured roughly two weeks behind baby #1… has anyone ever experienced this but both babies ended up being okay? I’m so heartbroken… we had literally just gotten the gender back and everything… everything seemed to be going so well and now this… I’m praying it’s was a mistake but I know I probably shouldn’t get my hopes up..