Here on the Gulf Coast, we can grow our bananas in situ. I don't have a lot of experience with container growing but once a banana starts to rot, it will eventually make its way into the corm. I would cut the pseudostem to soil level. If you see dark material anywhere on the corm, you need to cut it out with a sterile knife. Remove the corm from the pot, wash it off, and soak it in your copper fungicide solution for an hour. Repot in fresh soil and leave it alone until you see new growth. I would not even water it; maybe mist the top of the soil. Keep it in a sunny spot. You might get lucky but in zone 5a, you have a few more months to wait until you can bring it outside. This time from now until your warm weather is the critical time.

Steve