What is the ruling on a woman's prayer if, after performing it, she discovers that a braid of her hair was showing, and must she repeat it?

A woman's prayer is not invalidated if a part of her braid becomes uncovered without her knowledge, because it is not considered part of the severe awrah (private parts that must be covered). If the uncovering of the awrah is unintentional and minor, the prayer is not invalidated. Even if it were significant but for a short duration, the prayer would still not be invalidated.