He was perfectly fit, no signs of sickness at all. 1 day, layed up with slight fever, next morning he was dead.

This doesn't happen in just one day.

YES, and if your cat is not urinating it WILL die! Not trying to scare you, but it is a real problem that you need to take your cat to see a Vet ASAP!

Oh, I didn't realize he was already dead... sad day.

I'm a Vet Tech in training

Yes.

A blockage could have caused this. Either his kidneys could have been blocked by a large stone or his urethra could have been blocked by calculus. Either of these would have caused his own waste to back up into his body and also his bladder will burst eventually, which will kill him. This could have been prevented by a vet trip upon a visual of the extremely non subtle symptoms. Try and pay just a bit more attention to your pets next time. It may save a life...