Sad day.....

Well, another snow day. Teachers had to come into work today. As soon as I got there, I got a call that our dog has died. Can't figure out what happened. Can't find any injuries, he was just spitting up blood and died. The kids are all devastated, and I really don't know how to comfort them. I know its part of life, but its just so hard when your kids are upset. Any thoughts?