Horned Mountain Dragon Has A Section On His Tail That Looks Like It Is Deteriorating?

My horned mountain lizard has a section on his tail that looks like it is deteriorating. It is about an inch down from the end of his body. The portion of tail that follows it looks like it may be dead now. I just now noticed this and have no idea what could have happened to him. He has nothing in his cage that I am aware of that could have possibly fallen on him to injure his tail. I do have pictures. I do not know much about lizard diseases I have no idea what could be wrong …