Lucky Bamboo are Dracaena sanderiana cuttings rooting in water. Initially, they all had healthy roots to support the foliage. For some reason, the roots of one of the cuttings died and that stem is now finished and should be removed and discarded.
Let's hope that whatever caused that one cutting to die back will not occur with the others. There is nothing that you can do about it. Make sure you change the water regularly.