Vanishing twin?

so I went for my ultrasound and we were there for like half hour and the tech wasn't saying anything then she left the room to get the radiologist. I look on the screen and I swear I saw two husband was like there is no way. Well the tech comes in and said looks there was twins but one of them the sac is empty and they other one has a heartbeat. So looks like I lost one. I am sad but at the same time thankful I have one viable baby. I am in shock right now! Anyone ever experience this? Or had a good outcome and both twins survive?