That is frustrating. I'm sorry for your loss.
Don't beat yourself up, but with each failure I look to see if there is anything I can do to improve my techniques.
Could she have potentially gotten confused and taken up brooding residence in the yard? Some broodies do forget where they are at and take up brooding wherever after they leave the nest, especially if it is easy to get confused in the yard before they get back to the nest. Then as they sit and get cold, she may have fallen asleep due to the cold.
The only thing to do is make the broody hutch connected to a small pen so that they can still see their hutch when they get up to relieve but can't wander far from the area so when the broody hormones insist sit *NOW* they are prompted to go back to the nest.
Thoughts....
LofMc