My beloved cat died suddenly, and I’m in shock. What happened?

Dear Most Esteemed and Knowledgeable Kitties:
I had the shock of my life today.
My cat, Zoe, was only 5 years old and she was the most beautiful cat I ever laid eyes on. She was a large cat, not fat at all, but large and quite heavy. Yesterday, I picked her up and noticed that she [...]