Highly unlikely the bird died from chill unless it was soaked, totally, and the night was cold enough to at least frost the grass. In my best guess, your bird has been possibly accidentally but maybe deliberately jumped on or kicked by a larger bird, and gotten a fractured skeleton or other injury.
When banties and full size fowl are sorting out their hierarchy, banties often try to take a dominant role despite size, and even a gentle larger bird can accidentally kill a smaller one with a small kick. I've had a few birds with smashed skeletons, most survived because I kept them immobile while awaiting healing, but if something important got ruptured or bruised enough that would be the end.
It's not common for big birds to kill banties in my experience, in fact over here it's more common for banties to kill bigger birds, but accidents do happen. The bigger bird could have flown down from the perch and landed on the banty for example; even a small bird can hurt a large bird that way. I range banties with my larger fowl without problem year in and out, I've had a total of one bantam get a broken skeleton. She's fine though.
