Around 1 AM I stepped out onto the porch during my closing up of the house for the night and heard BlackJack calling to me from the area below the porch. The house is a split level, and the bottom third is rented to a guy named Joe (who is away currently.)
Grabbing the flashlight, I shined it down on Joe's gate, and saw Jack lying on his side. He was calling for me very strongly. The Sister-Unit and I ran down the steps and scooped him up. His hind legs weren't working, and they felt cool to the touch.
Glen is at the vet's with him now. We thought he might have fallen off the roof and hit his back on the gate, but the tentative thought of the ER vet is that our 8 year old cat threw a clot from his heart that traveled down to his legs, cutting off blood flow. Maybe... but he's never shown sign of cardiac distress before. He's very robust.
Still, I know people can died suddenly of heart failure when they seemed as healthy as a horse. We love our tuxedo kitty, and cannot bear to lose another cat this year. (Elsa and Ivan passed within two months of each other...) Please send BlackJack your well wishes. I'll update when I know more.
Edit: BlackJack's heart disease is very advanced. Fluid has built up in his lungs and around his heart. He has no circulation in his hindquarters, and will likely continue to throw clots. There is nothing that can be done to improve his situation, and the painful decision has been made to let him go and join his sister Winky across the Rainbow Bridge. We love you, Big Guy -- I already miss him horribly...