when you go to give the nut a tap with a hammer try and use some thing like a copper hammer it should not damage the nut but still give a decent hit . if you do end up needing a nut you need to know thread dia and thread pitch ,eg ,1,1.25,1.5, 1.75 you will need to get hold of a thread pitch gauge to measure the thread, when you do get it all free and moving use a good anti seize to stop it from happening again , any way best of luck.