happens often, the stud is kept from spinning in the hub because it is knurled, now it is not. You have to cut it apart. I use a die grinder with a cut-off disc and attack the lug nut. If it is recessed you may have to remove knuckel to access the back side and tack weld the stud in place then remove the lug nut. The stud will need replaced, and if the hole has been reamed out by the stud spinning the hub will also have to be replaced or you will want to tack weld in the new stud.
Always replace all studs on a hub if one needs replaced.