It could have been much worse. A battery that froze solid can develop some interesting issues when used after it thawed, such as thermal run-away (or interesting exothermic episodes, as we call it, but funnier to watch in a reinforced calorimeter than in your driveway), or short-circuits (lots of smoke if you are lucky, otherwise a fireball). Batteries have operating range for a reason, and that operating range is exceeded routinely in winter and summer, even in nominally temperate regions.