We don't really know enough to say, but it is possible that the plant is overwatered and is rotting. Is the stem soft or squishy? If so, you need to remove the soil, cut off any rotten parts, and store healthy pieces in the shade until any cuts heal over. At that time the pieces could be replanted, and kept in the shade until rooted.

It is also possible that the plant has mealybugs or other parasites, and if so it should probably be discarded. Scrutinize the plant for white or powdery specks on leaves and in joints.