This looks like crown rot or root rot.  It is possible that if the very top part is not at all mushy, you could take the firm healthy part and insert it slightly into the top of a clean pot of sterile soilless potting mixture and it might root.  It looks like there is a tiny off-shoot in the pot and you might repot that as well.  Crown rot is caused by overwatering or allowing water to stand in the pot.  The soil is probably filled with pathogenic fungi and should be discarded.  https://extension.umd.edu/hgic/root-rots-houseplants  http://extension.missouri.edu/p/G6560    https://www.ndsu.edu/pubweb/chiwonlee/plsc368/student/papers01/pneisen/Propagationofcactiandsucculen...  vw