If your vehicle will not start and you are seeing a security or key-shaped light flashing on your dashboard, then you may potentially need a variety of repairs. Some of these can be done on your own, while others will require taking the car to a dealer. The security system is an immobiliser, which is equipped in many cars since the late 90s and early 2000s. The immobiliser uses a microchip implanted in the ignition key and will only start the engine if the microchip is in the key that is placed in the ignition.
Try your spare key. It could be that the primary key you have been using needs to be reprogrammed, which can be done by contacting a dealer. A spare key will have the same microchip technology, and because it has not been heavily used, the microchip will still be good on that key. Depending on your make and model, you might be able to save some money and call a car locksmith and ask about services for reprograming keys if the spare key does not work.
Turn the key to the "On" position in order to reset the pass lock system. Wait for fifteen minutes with the key in the "On" position. If the security light disappears, turn the key to the "Accessories" position and wait for a couple of seconds. Turn the key to the "Off" position and take it out from the lock cylinder. Wait for a couple of seconds and turn the car on as you normally would.
Install a relay that bypasses the security system if your car is using a remote starter. Contact your remote starter dealer or installer to ask about the relay. It is possible to also install the relay if you do not use a remote starter. Install the relay by finding an electrical circuit diagram for your vehicle and locating the security immobiliser circuit. Cut the circuit with wire cutters and solder the bypass wire into the circuit. Test it by turning your car on to make sure that it works.
Take your key to a dealer or a hardware store if you have recently lost a key or replaced the ignition switch. Only some hardware stores are able to duplicate microchipped keys while nearly every dealer will be able to help by making a duplicate key with a microchip.