Eliza has passed out from sheer exhaustion, both physical and emotional. In Chapter 7 she has made the legendary leap across the ice to safety, her son Harry held tightly to her arms.
She is tired and hungry and cold and miserable and afraid; afraid for her child, herself, and her husband from whom she has been parted.
In Chapter Nine, her body simply shuts down and she passes out. Discovered by the kindly Mrs. Bird and her husband. Stowe writes:
He laid down his paper, and went into the kitchen, and started, quite amazed at the sight that presented itself: - A young and slender woman, with garments torn and frozen, with one shoe gone, and the stocking torn away from the cut and bleeding foot, was laid back in a deadly swoon upon two chairs.