One solution is to drain off a gallon or so of what's in there and just add a gallon of straight anti-freeze. Then check it with a hydrometer to see where you are. Next, when the temperatures moderate, I'd flush the whole thing and start over with new anti-freeze/water mix to the correct proportions. Don't be cheap, this winter is predicted to be much colder than normal (look for how that conclusion is reached by the monitoring of late fall snow cover in northern Europe). That forecast seems pretty accurate so far. You probably have caught it in time but a frozen block is too be vigorously avoided.