I have a cactus that is going hollow on one side.... Please Help?

I had a cactus that was growing well, but about two weeks ago I knocked it off a stand and the new growth on the top broke off. I then took to my basement and let it sit for several days. I bought some dirt and re-potted it. Now half of it has gone hollow and flimsy. It's like you go punch a hole in it. Please help me to save this.......