Realms Keep Dissapearing

So, just yesterday I was playing with one of my friends on their realm. It was late, so I went to sleep. The next day, I got on the computer to play with them some more, and their realm was no longer on my list! They were not on trial realms, they bought it. I don't understand what happened. This also happened a while back when I was new to minecraft, but I didn't think much of it. But, since it happened again, I'm not sure what is going on. Any explanations?