It looks to me like some kind of fungal rot, mabe phytium, rhizoctonia, fusarium or any other soil disease. They kill the stem/roots tissues and the rest of the plant can't get any water or minerals, so the tissues with more water inside suffer the most. Maybe been kept in a closed space for a while made the environment more humid, and easier for the fungus to invade the plant.
I think it's nothing to be done at this stage. If you have more plants, keep them away from this one. You should try to use Trichoderma in order to save the rest and/or prevent more infections, you can use some chemical fungicide as well.