All the above are correct, but if you got no heat or welder to hand look like you could double nut that also.
before you go for the big un Give it a good smack on the top of the nut and start by trying to tighten a touch , then try to loosen, tighten, loosen.
if it starts to move dont get too excited and take time mate, loadsa of oil and loads of tea.
if that dont work i would try heat , final resort IMO would be an easy out
jth ( have to do this **** all day / everyday for ******s that snap **** and walk away at work) i feel you pain.