Since the car engine was frozen, temperature will be extremely low outside. Tow the car to a garage which naturally is much warmer than the outdoor environment. Use a thick rope for the job.
Once the car is parked in a garage, lift its hood. Give the car engine a close look and see what parts were frozen. Scrutinise the car thoroughly because when the frost melts, you will know that all the car parts that were frozen are now unfrozen.
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Light a propane heater with a lighter and place it at a safe distance from the car. A distance of about five to ten feet will do. If you place the propane heater too close to the car, chances are that excessive heat will ruin paintwork on the car’s body and may even disfigure the tyres.
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As heat from the propane heater increases the air temperature, frost on the car engine will slowly start melting. You need to be patient during this time. Since you are not directly heating the engine, it may take anywhere between four to eight hours to unfreeze it, depending on how much frozen the engine was. While the propane heater is lighted, make sure that you do not leave the garage. If you have to, turn off the heater before leaving.
Keep looking at the car engine at regular intervals. When you think that the frost which had frozen the engine has melted away, turn off the propane heater. Closely inspect main engine parts such as the coolant reservoir, the engine block and all radiator hoses. If you think that some of the parts are still frozen, light the propane heater again and wait until all the parts are completely unfrozen.
Finally, try to start the engine. If you are lucky, no serious damage would have been done to the engine and it will come to a normal start.