It's impossible to know for sure what happened without more information, but here are some possibilities:
* Overfeeding: Goldfish are notoriously overeaters. If the goldfish ate too much, it could have led to bloat, which can be fatal.
* Bloat: Even if you didn't overfeed, your goldfish may have had bloat from other causes, such as stress or a change in diet.
* Internal parasites: Parasites can sometimes cause problems with digestion and even lead to death.
* Water quality issues: Goldfish are very sensitive to water quality. Even a small change in ammonia or nitrite levels can be fatal.
* Sudden illness: Fish can develop illnesses very quickly, and sometimes there's nothing you can do to prevent it.
What you can do:
* Review your feeding routine: How much food were you giving your goldfish? Were you feeding them once a day or more?
* Check your water quality: Test your water for ammonia, nitrite, and nitrates. If any of these levels are too high, you'll need to do a water change.
* Consider a fish vet: A vet specializing in fish can help you determine what happened and if there are any steps you can take to prevent this from happening again.
I know this is a difficult time. Remember that fish can be delicate creatures, and sometimes things happen that are outside of our control. Do your best to learn from this experience and make sure your remaining goldfish is healthy and happy.