That's awful Lee... It's scary how quick they can go down hill...
We had a rescue Jill (Snowey) who came to us at 7 years old. She seemed as fit as a fiddle, and was a proper maniac, literally bouncing off the walls, and the same as you, one night, before going to bed, she was all over the place, having ferretone, and playing like a mad thing, and the following morning, stone cold.... Same thing, vet said it was a heart attack... never saw it coming.
Sweet Dreams Vic and Micky, you had a great time doing what ferrets are meant to do. Have a ball at the Rainbow Bridge.