I'm very sorry for your loss. I started to post a response last night but thought at that point it would have been to late. I agree with mls regarding spooking a down horse. I would have suggested stacking haybales along the horses back to help keep him in an upright position. Then I would have suggested a backhoe w/ two tow straps to get him up. I used that technique with my old black horse and was lucky enough to have it work. I'm only posting suggestions now in case someone happens along that is having the same issue, maybe they can learn from your loss.
One thing you should know is that if you took this horse in as a rescue and brought it back to health and gave it love and care over the last year, then you did that horse the biggest favor and it died knowing it was loved. That is what matters.
Sometimes with rescues, you don't know what damage is on the inside from abuse and neglect. I'm thankful for people like you who take in animals in need. I'm sorry for your loss.