Hopefully it is not frozen solid, but more "slushy". If it freezes solid it might damaged the plastic tubing from the pump to the squirt nozzles.
You mentioned a garage, I'd try to use some sort of portable heater to raise the temp in the garage for a day or so to unfreeze the fluid, then I'd use a turkey baster to empty the rest of the fluid. Replace with the lowest temp rated fluid.
You could even try using an old electric blanket wrapped around the tank to heat it, but that will not help thaw the fluid in the lines.
I seem to remember the Peak stuff being rated to -20 deg. I don't think I've seen anything rated any lower than that.