The little chick may have suffered injuries by being stuck at the bottom of the box. About all you can do now is supportive care. It sounds like you have her isolated from the rest of the group? Because you do not want her getting picked on in her weakened state. You may want to use a towel in a separate cage, which would provide better traction. You can even form the towel into a cup where she could sit. It might be more comfortable than an actual cup. You could feed her some mashed up hard boiled eggs. i give that to all my new babies and they seem to thrive on it.
Hope she pulls through.
Hope she pulls through.