Yes, your cactus probably has been overwatered. This caused the plant to be soft and mushy. There could be root rot also. You can always check the roots and repot in fresh soil if they are okay. Iβm not sure much can be done to save it. οΏΌπ
Looks like stem rot β if itβs an important plant to you Iβd just propagate it. You know, cut off the part that isnβt black, let it callous over etc.