Hello Chris, and

!
That is very sad about your little goldfish, and unfortunately, for us to even slightly figuring out what might have caused it, we need some more info. I'll put your post into the disease/diagnostic section, and there is a box on top of that section with all kinds of questions.
Try to answer as many as possible - that gives us a better picture of what might have happened. More importantly, if you have any remaining goldfish, you need to keep an eye on them to make sure they are okay as well.