When starmalachite and I came home from seeing friends at a New Year's gathering, I found our elderly white cat Satin lying on the laundry room floor, back arched and front paws repeatedly thrashing. She didn't respond to me or show any sign that she was even aware. Our best guess is that she had a stroke (she'd apparently had a less serious one a few weeks ago, and was recovering from that); the vet we talked to concurred. It took me a few minutes to fully accept it, but there was clearly nothing to be done but let her go, and so we brought her in to the emergency vet to put her to sleep.
I can't think of anything much to say beyond what Crystal said in her entry. We'll both miss the little furball.