oh wow - I'm so sorry !! 5 is so young but at any age that would be devestating.....I can't help you on why that could happen but it's not normal for any dog to suddenly die like that...bless his heart -
I hope your vet can give you answers ... if it were me, I think I'd want to know - eventually...after I got done being a curled up ball in the corner crying.....
My condolences to you.