Any one every had a vanishing twin?

so I went for my ultrasound at 7 weeks 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 every had this happen or have a positive outcome and possible the other baby was just behind in growing?