Your pot is too big, the soil stayed too wet and the roots rotted off. All you can do now is try to re-root it. Pull off the bottom few rows of leaves, get out a sharp knife and cut the stem. You are looking for healthy green tissue, no brown, inside the stem. Keep pulling off leaves and cutting until there's no brown in the stem. Spray the cut with a fungicide and 'plant' it in a small pot. Don't bury any stem, just make sure the cut end makes good contact with the soil.
You can start new plants with the mature healthy leaves you cut off. Just make sure they aren't showing signs of rot.
My largest AV plants are in a 4 inch pots.