This happened to one of mine, rotting from the outside in. I found out an animal had been peeing on it repeatedly - but it took me probably a couple of MONTHS before I figured this out. I kept cutting the exterior layers until I would get down to good tissue and I replaced the dirt around it twice. I put citrus peels and black pepper around the base of the plant, which stopped the peeing. But by the time I figured it out it it was too late and the plant rotted to about and inch or so below ground level. However, as of right now there are 9 pups peeping up a few inches away from the base of the rotted corm. This was a huge surprise to me, so there may be hope!
Good Luck!
~JaNan