I'm sorry to tell you that when the stem of a Dracaena marginata shrivels and dies, as yours has, it is because the roots have already died and are no longer able to support the plant. Unfortunately, there is no salvaging it at this point. It is very understandable that you are attached to this plant. Many folks get attached to their plants. But plants don't live forever.
Why did this happen? The problem started when it was repotted into a larger pot. When repotting is done unnecessarily and/or incorrectly, a lot can go wrong. In addition, a larger pot with more soil increases the chances of inadvertent over watering because the added soil stays moist for longer than necessary.
Many folks make similar mistakes, so don't be hard on yourself. Try to learn from the experience so you will have more success in the future.
