at -20C it's possible that the freehub simply froze, in the sense that the lubricant inside got too stiff and bound the mechanism. Did you bring it inside to thaw. If so, that may be all that's needed.

OTOH, if the mechanism is still locked up try forcing it backward with the wheel out, and if you can get it to spin even slowly, odds are that your winter commuting allowed water inside and you have some rust or die related issues. Flushing often helps, but the method depends on the freehub. Some have holes in the side under the cassette where you can introduce solvent or oil. Others have ports in the end cap (not sure if any of these are still made), and yet others are almost impossible to flush while still attached. Those can be removed, and soaked in kerosene or diesel fuel (#2 heating fuel) , then drained and oiled. For the type of winter riding you're doing, I suggest a light Synthetic which won't overly thicken in the cold. Mobile 1 is a good choice, though when it warms up you'll want something thicker.