My compost pile is frozen
It is not dead and it does not need fixing. A frozen pile is paused. It will restart on its own within a week or two of the thaw, and the freeze-thaw cycle actually helps by rupturing plant cells. Below: what causes frozen compost, how to fix it today, and how to stop it happening again.
What not to do
Do not turn it. You will only release whatever warmth remains in the core.
Do not add water. It is not dry, it is frozen.
Do not add activators. Nothing you add will overcome the temperature.
What you can do
Keep adding material on top. It will freeze, stack up, and be waiting when things warm. Cap the pile with leaves or straw for insulation.
The upside
Freezing ruptures cell walls in plant material. When a pile thaws, that material breaks down noticeably faster than it would have done without the freeze. One reason spring piles often take off quickly.
Related
Seasonal context: winter composting. Hub: seasonal composting.
Further reading on frozen compost
Independent sources worth reading alongside this page: EPA, Composting at Home · Cornell Waste Management Institute.