That sounds just like what happened to a yearling of mine earlier this year when he had colic and died. It was absolutely horrible. I don't know where you're located, but I send my colt to a diagnostic center to find out what caused the colic, to make sure my other horses didn't get into it. It doesn't cost much, and they'll also dispose of the animal afterwards, at least at the University of Kentucky. My vet made it out about 5 minutes after he died, but I caught it late, after I came home from work. It's devastating at first, and you fret over things you could have done differently, but you'll start to feel better eventually.