My 3 yr old cat died but the cause is unknown...any ideas?

My 3 year old cat started displaying some weird behavior (not wanting to go outside, no treats, no nothing). I decided to watch him throughout the night and by the next morning I found him on the ground, he was still breathing but there was blood coming from his and he was really cold. So I rushed him to the vet. He said it may have been due to a blocked urinary tract or something like that since he had been neutered. Anyway, they called me an hour later saying when they put in his IV, that he went into cardiac arrest and died. We had an autopsy done to put my mind at ease since he was so young but the vet found nothing!! No idea what caused his death or what happened. Any ideas??