In theory if the coolant freezes it should push the freeze plugs out of them block and save your engine...........in theory...
It isn't expensive to replace a freeze plug yourself, it is just a metal cap that presses into the hole, that way when the coolant expands it is just pushed out of the block, the problem is that some of them can be a real pain the ass to replace. Usually you can do it without removing the engine from the car but some of them require you to be a contortionist
How cold did it get? Nobody runs straight water in a radiator, you need the Anti-freeze to keep your water pump lubricated. As had been mentioned earlier you would have to get pretty cold to actually freeze it, typically I think most people up here in Alaska run their coolant/water at about 50/50 or 40/60. So I have a feeling that your car didn't freeze solid.
If you did pop a couple of freeze plugs consider taking the time to replace them with block heater plugs so that you can plug your car in a night, another good investement is a coolant heater, typically you just spice it into your heater hoses. The nice part is that when you go to start you car after the heater has been pluged in you get hot air right away since the coolant is allready warm. No waiting for it to warm up.
Just to warm up your engine since it is allready cold, get a drop light or two with a 100 watt lightbulb and leave it on under your hood overnight, you may consider putting an old blanket over the hood to hold the heat in. If there is a puddle of coolant under it hopefully you only pop a freeze plug, otherwise you may have some $$$ to spend for repairs.
If you have a torpedo heater(reddi heater) you could also point it under the car for a half hour or so and warm it up that way.
If you car still doesn't start, or it starts and starts spewing fluid/steam then you need to shut down and figure out where it is comming from....think on spending $$$ again.
Just out of curiousity how cold did it get? and what type of car/engine are we talking about.
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