Welcome to Aquarium Advice, and I hope we can offer some help here.
There are plenty of things that can kill a fish, and the most common ones are related to water quality, but it could have had an illness or just died of old age. I think all of us have had a mysterious fish death at some point or another. Anytime I have a fish die I check all of the water parameters I can, and keep track of that information so I can compare it with last week's routine check and see if anything has changed.
We need more clues though, in this case, so we need more info, like Smonkey15 mentioned. How big is the tank, how often do you feed, how often do you do water changes, tankmates, and levels of ammonia, nitrIte and nitrAte. There are a lot of things that can play a role.