Damage is possible from freezing coolant. Most of them are good to around -30 something. You probably won't know till you get it started.
I'm not sure what you mean by frozen. The coolant is frozen, or the engine simply won't crank? Anyway, keep the block heater working. If it's possible, you might add some external heat - cautiously. I mean, don't build a fire under it.
Take the comment on battery charge to heart, if the engine isn't cranking. Batteries lose lots of capacity at low temperatures.