If it turned around completely already without coming loose then you may be screwed.Pun intended!It will twist the antenna wire off inside the fender and will need replaced.Real pain in the arse too.
For proper loosening use the correct tool which is a 8mm open end wrench and you have to sort of quickly flick your wrist about a 1/4 turn to try to loosen it. If the whole base down inside the rubber wants to turn ,I would put a drop of penetrating oil near the base and try again the next day! Use of a dielectric grease is recommended for these 5mm threads on the antenna stud.
So not sure when or how but it has been snapped off clean at the base ! Thinking i might be able to back it out reverse thread but all it did was push it lower down . Only option to replace the base or ? :ybrick: