Indoor ferns if placed outdoors before the weather warms can indeed get frost bite and the cure is simply to trim off any damaged or brown parts of the plant. Your fern should make a full recovery. You did mention that the fronds were brown underneath and this could be a sign of the old leaves naturally dying back to make room for the new fronds on top. If your fern is sprouting fresh leaves from the center of the plant I would not worry and just trim the bottom or brown sections. Make sure to bring your hanging plants indoors if frost is forecast and also protect them from the sun. Spring is coming, but not quite here yet. Keep growing, Marianne Binetti