What happened to my beautiful butterfly Koi?

Hello everyone, I came home from work today and saw my beautiful butterfly koi was floating and dead. This is the first time happened to my koi so I do not understand what caused it. I had had the koi for 12 years and she was healthy and happy. If you know what might have caused my koi to die, please leave me a comment. Thank you Thank you for watching. Stay safe and do what you enjoy and love to do in life.