I'm guessing it has root rot. I would replant in fresh media immediately. If the media is badly decomposed it will hold too much water for too long and as a result the roots suffocate and then rot. You want your media to drain well so sufficient air can get to the roots. Many folks use bark, but I prefer coco husk chunks, charcoal, and perlite mixed in a 4:1:1 ratio.
When you repot make sure to cut off all the dead and rotted roots with sterile scissors or knife. Healthy roots will appear white with green tips, or green. Older roots can be tan to brown but as long as they aren't squishy you can leave them. Dust all your cuts with cinnamon and repot. Then you wait patiently.