If your car has an anti-theft device, it may occasionally go into theft mode. This generally happens when the wrong key is used in the ignition or if someone tries to break into your car. When this happens, the engine will not start. To get your car out of theft mode, you'll need to use the correct key and reset the anti-theft system.
If you have a transponder key, hold it up to the ignition and turn it on. This should reset the anti-theft system and allow you to start your car. If you don't have a transponder key, you'll need to find the override switch. This is usually located under the dash or near the steering column. Once you've found it, flip it to the on position and then try starting your car.
If your car still won't start, there may be a problem with the alarm system itself. In this case, you'll need to take it to a professional for diagnosis and repair.