Long term? Heat tape - leave it in the pipe, leave the plug at the bottom, then it up so the water doesn't contact the plug.
Short term, I've tapped it to break a thin layer of ice inside, unscrewed it, and brought it inside to melt, the reattach.
Is it frozen SOLID? Plug the top so it doesn't refill, let it spill over the side. Got one of those BIG torches? Start at the bottom.
Or just wait. Spring will come.