Your problem could be from various problems, you may have a hole in the radiator from something that punctured it, it could be a hole in one of the radiator hoses or the hose could have come off, you could have had a frost plug pop slightly or completely from a weak coolant solution and cold temperatures or even worse a cracked block from the same problem. The first thing that you are going to need to do is get your head right into the engine compartment from the top or bottom using a flashlight and find out exactly where the coolant is leaking from and should be able to help you a little better.
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Your problem could be from various problems, you may have a hole in the radiator from something that punctured it, it could be a hole in one of the radiator hoses or the hose could have come off, you could have had a frost plug pop slightly or completely from a weak coolant solution and cold temperatures or even worse a cracked block from the same problem. The first thing that you are going to need to do is get your head right into the engine compartment from the top or bottom using a flashlight and find out exactly where the coolant is leaking from and should be able to help you a little better.