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Hello, my name is Greg. So sorry for your sudden loss of your cat. There are many reasons why cats can die suddenly... since you did not notice any issues with the cat beforehand, it was likely due to a heart condition. For cats this is common and without signs before the issue occurs. It may have been cardiomyopathy - however, you cannot really be sure without taking to a vet for an autopsy.
CLICK HERE for more info on this topic that might help. Again, my deepest sympathies on the loss of your family member.