And then come down and eat everybody?
One has to presume that her brain had been sufficiently damaged to prevent reanimation. How that came to be is anybody's guess. We don't know anything about her struggle with whatever ghoul dined on her, nor who else might have been present, nor what happened to her killer. One might consider it to be a plot hole; I prefer to think of it as an unresolved mystery.
When Ben is dragging her to another room, we can see that her face is completely intact. But that's George Romero's fault, he picked the worst possible angle to film it.
People says that her husband or someone else shot her in the head. Her face IS kind of pealed off or whatever, i think. But you are right, it always bothered me she was upstairs and no one was worried. They don't know a shot to the head is the answer right away, yet they seem fearless of her corpse. The first time I saw this it was before shot-to-the-head zombie movies were abundant and I didnt know if she would reanimate or not and it worried me the entire film.