Unexpect death of young cat?

i swear, this just happened to me saturday night. before i went to work my cat was laying down. he was never sick and no heath problems. i then came home from work and i found him in his cat bed curled up asleep but he was dead. his eyes were open a little but sometimes cats sleep with their eyes rolled back. but he was gone. i looked around the house and their was nothing that he could have gotten into or nothing. i too, was unable to get to a vet becuase this happened sat night, every vet was closed sunday. so i ended up burying him. ive also read about possible being heart attack or heart worms..but its all so confusing for me. he was only 3 so he was a very young kitty taken from me. i hope we both find the answer we are looking for for our sudden deaths of our loved ones...