it happened to me last year,when i filled my car with normal water and brok my starter motor belt when tried to start engine... recovery man towed it to home and suggested to let it un-freez and drain, then get checked if frozen water damaged the engine/radiator etc (as water get expand at freezing point)....
luckily it was only the matter of changing "core plug" a total £4 job to get it replace by a local mobile mechanic.
and filled with proper anti-freez, fully concentrated.
core plug is the light weight metal cap somewhere undernith car, which gets crack (avoiding damaging engine and other expensive equipment) and gives space to water to get expand.
Imran
December 2010