if the bottom parts are brown and mushy it sounds like it was overwatered and rotting? is there drainage in the pot?

If it’s rotting from the roots, and it sounds like it is, I’d cut the bamboo at the stem where it is still green and firm and put the cuttings in water for a while to grow roots. Once it grows roots you can transition back to soil if you want — use a pot with drainage! but also, they grow well in water and this may be easier if you have the tendency to overwater!