They rotted from too much water. If you bring banana plants inside your house or garage for the winter, you must cut back on watering significantly.

i thought that i did cut back on the water, after i brought them inside. i do have 3 other banana plants that stay indoors all year.
i had brought the Basjoo inside cause they never grew that big, and when i have tryed to overwitner other plants in the past they had always died.
So i figured id over winter them in the house this year and see what happens this summer.
cause to tell the truth, im am bad at watering my plants indoors, if anything id think i dont water enough.but thanks for the responce

This happens sometimes with bananas. Probably a fungus.
The plant has had some (unvisible) damage to the stem, a small crack by replanting can be enough, and a fungus could be developed into the stem.
If you cut the stem fast enough, below the brown spot, low enough to be fully green, you can save it otherwise it will surely die.
It happened last year to a French Horn at my place after moving.
Simon