Hen died instantly

Hi, we have had the worst day, asked hubby to put cockerel away in shed so I could let my 3 new ex batts out for a little while as they are still bald, by the way I have 9 bantams & the 3 ex batts, anyway cockerel ended up getting through fence into neighbours garden & was savaged by dog, our chickens have no fear of dogs as we have 3. This was bad enough then we are sat in garden waiting for hubby to dig grave & glanced at 1 of my bantams & thought she didn't look well so picked her up & she sound like she had a wet rattle, immediately on picking her up blood started pouring from her mouth & started fitting, she died within a minute. We are all absolutely devastated as they are basically our pets. Kids inconsolable. Do you think it could have been shock? Although we have been keeping chickens for a couple of years now, they have all been fit & healthy so have no idea why she died so suddenly!

it could well be shock.
Keep an eye on the others, check them over for illness signs, eyes/nose etc.
check boundries and plug any gaps.

What a horrible thing to happen!  Patch up that hole in the fence right away and I'm sorry for this awful experience for you and your children! 

Thank you all for your kind words, got up this morning & hen house seemed very quiet, was dreading going in but they all seem ok, it was only the cockerel that got through as we were all outside, admittedly I had to run indoors when I heard that he had caught it, unbearable! Trouble is we have a thick fir hedge & the fencing is behind that, now fixed but my husband just couldn't get through in time. We are now looking to also put fencing this side of hedge. Thank you all again.