There are many, many different possible reasons why a car will not start. There is no way of guessing whether your car doesn't start from a common cause, or from an unusual cause. Because of this; it would be much easier to give you a pertinent answer if you had included more information in your question. If, for example, you had stated whether you can either hear the starter motor running or could see the fan belt turning when you turn the key to the start position; then I would know whether this is a problem with the starter, or with the engine. If you had stated whether this car has a manual or automatic transmission; then I would know whether you could then check whether the engine has locked up by pushing the vehicle in gear (only possible with a manual transmission). If you had said how old the battery is, and explained the way in which the battery was checked, I would know whether there is still a possibility that the battery may be the cause of the problem. If you had stated whether the gas gauge was reading low the last time you drove the car; I would know whether it may simply have run out of fuel. If you had stated whether the car had recently been worked on; or had overheated; or had been running roughly or stalling; I would know where to go from there. If you had explained whether all the electrical accessories appear to still be working, or whether certain items have stopped working, it would help me to know about the condition of the power distribution system. But as it now stands, I know next to nothing about the situation with your car. And that means that; in order to help you, I would have to write a mini textbook here on sorting out starting problems. I hope you can now see why I need more information.
Instead of wasting all kinds of time trying to cover every conceivable possibility; it would be simpler for me to ask you to answer some basic questions about the car:
A> When you turn the headlights on, and try to start the car; do the headlights become dim, or do they go out; or do they stay the same brightness; or do the headlights not work now?
B> How many miles are on the car, how long have you owned it, and has it been giving trouble, or been worked on recently (and if so, what were the details)?
C> When you turn the key to the "start" position; can you hear the starter motor running? When someone looks under the hood at the motor from the passenger side, while the key is turned to the start position; do the fan belts turn?
D> Can you hear the electric fuel pump running (making a humming or buzzing noise coming from the rear of the car on the drivers side) when the key is first turned to the position where the dashboard warning lights come on? Do all the electrical accessories work?
E> Does the starter motor sound like it usually does when you try to start; or does it now sound like it is running faster or slower than normal; or does it make no sound at all; or does it click loudly once, or make a rapid series of clicks?
F> Do you know whether the timing belt has ever been changed; and at what mileage?
G> How many miles has it been since the car was last tuned up; or at least had new spark plugs installed?
H> Does the car have a manual transmission or automatic transmission?
I> How old is the battery; and how was it checked?
J> What level does the gas gauge read when you turn the key on?
The more information you provide, the better answer I can give you. Please post your response underneath this one; by clicking the "answer this question" button at the bottom of the screen. If you post your response to this discussion as if it were a new question; it will automatically be deleted. In order to be seen by us, your response has to be put here. Thank you for your cooperation.