I agree with Virginia Jim. It will almost certainly sprout from the roots and I'd cut up a few inches from the rotten spot until I encountered healthy tissue. Let it sit in partial shade unpotted to see if it heals or continues to rot. If it heals I'd replant in well drained medium in a couple of weeks. You might want to check for a large insect larva (agave snout weevil) in the rotting section. Just to be sure it's cold and wet that caused this problem. Particularly if you bought this as a trunked specimen.