A sudden death like that with a pool of blood is quite strange - is there any possibility that she could have been attacked by a predator?
If you are certain that it wasn't a predator attack, had there been any change in her behaviour over the last few days, or is it possible that she had access to something that she shouldn't have eaten? (I'm thinking along the lines of rat poison, which works by causing rodents to bleed internally)
It is very hard to try and guess what might have caused her death - the best thing would be to get her necropsied by a vet. If you are going to do that then you need to bag the body up and keep it in a cool place (but don't freeze it) until you can take it to the vet. Here in France my local vet charges 15€ for a basic necropsy. She isn't an avian specialist by any means (in fact she learns a lot as she goes along each time I take a sick bird in!) but she can see a lot more than I would be able to, plus she can do blood tests if needed.
I imagine your boys are going to be really upset, but it is all a part of keeping animals, albeit a sad part. You can stress to them how happy your girl was to live with you, and how you all looked after her so well and gave her such a wonderful life.
As well as your boys missing her, your remaining hen will also be feeling lonely. You should consider looking for a new companion for her, as chickens are very social animals, and she will not be happy to be alone.
Sorry for your loss - I hope that you find out what the cause was, so that you can hopefully avoid it happening again.
