Car won't turn on, what could it be?

i know little about cars, sorry if this is confusing. When I turn my key halfway, the lights come on and the radio works but when I turn the key, all the way, the car does not start. The engine won't start. Additional info: It worked yesterday but today, it is much colder outside (almost freezing...well... show more i know little about cars, sorry if this is confusing.

When I turn my key halfway, the lights come on and the radio works but when I turn the key, all the way, the car does not start. The engine won't start.

Additional info: It worked yesterday but today, it is much colder outside (almost freezing...well maybe it is freezing).
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