As you may have gathered from the responses you've gotten so far, there are a lot of different reasons why your car might not be starting. It will take some troubleshooting to figure out what's going on with it.
Let's start with a better description of what it means when you say it won't start. What happens when you put the key in and turn it to Start? Does it do nothing, do you get a click-click-click noise, or does the engine crank over?
Depending on how you answer that question will determine what we look at next. If it doesn't turn over you might be looking at a dead battery or maybe bad terminals, a dead starter or even a frozen engine. If it does turn over but won't "catch" and start, it could be fuel, air or electronics issues.
Let us know what's happening with it and we'll see what we can figure out from there.