The same thing sadly happened with Benjamin James aka BJ the Gray Meezer.
He was fine ,then one day he started to moan, and I put him on the bed when he circled, collapsed and passed away in seconds.
Our Vets sure that it was a heart attack.
Theres sadly nothing can be done about Our Cats who suddenly pass on.
But I know that Luckys in Paradise now, and hes smoothing the way for his family and Pet Friends so there will be a place for you all to reunite.
Maybe at BJS Barbecue Buffet.
One Fine Day.