or heart muscle leading to sudden death. Was the cat pretty hyperactive? Thin? This condition used to be more prevalent than perhaps it is now and may have been genetic and/or inherited nutritional deficiency related.

There is another type of cardiomyopathy (dilated), but it generally shows more symptoms before (tiredness, breathing difficulty).

If this is an inside cat, had been healthy, and no visible signs of trauma or bodily discharge then things like electrocution or toxicity are unlikely.

So very sorry, blur. That is so hard.