He had passed away. The vet said he had Hypertrophic
Cardiomyopathy. In other words, he had an undiagnosed heart disease, and
apparently had a heart attack and died.
We are, to say the least, in shock.

Oh my god. How incredibly shocking. It must be hard to believe that he's gone.
I had a cat go in half a day. I walked around in a daze for two weeks. I just
could not believe it. I will keep all of you in my prayers. Take care.