Yeah, a shorted cell in a lead-acid battery is usually game over. Minus 20C definitely puts a strain on them. Unfortunately, there's no "high-tech magic" cure for a physical short. The plates in that cell are likely touching, and that's not something you can easily fix. You're probably looking at a new battery, I'm afraid. You could try a desulfation cycle, but honestly, with a shorted cell, it's highly unlikely to help. Consider it a lesson learned about cold weather and batteries! Maybe get a battery warmer for next time if those temps are common.