Oh, my Heather - how sad and shocking when they leave us with no warning and no signs of a problem. One of my co-workers daughter's show Quarter Pony just recently had almost the same thing happen. Everything was fine that night - after her normal work out, they bedded her down in her stall, fed her and the next morning when they went out to feed, she had died. No warning, no signs of anything wrong.
My heart goes out to you