What might cause sudden and (seemingly) painful death of a cat?

My grandmother’s cat died today. She wasn’t especially old, maybe 11 or 12 and seemed to be in good health, though my grandmother said she seemed slightly “off” the last two days or so. She was an indoor cat and the only pet in the household, my grandmother’s companion. She would often sit on the back of the sofa behind my grandmother, and then hop down into her lap, or climb over her shoulder and sit on her chest.
Today my grandmother said that the cat was on the back of the couch and she “screamed” and jumped down into her lap, and by the time she landed she was dead.

What might the cat have died from?