Mustang.09.car.wont turn on check.brake system

Check the battery. If the battery is dead, it will need to be replaced.
If the battery is not dead, check the starter. The starter is responsible for turning the engine over. If the starter is faulty, it will need to be replaced.
If the starter is not faulty, check the ignition switch. The ignition switch is responsible for sending power to the starter. If the ignition switch is faulty, it will need to be replaced.
If the ignition switch is not faulty, check the wiring. The wiring is responsible for connecting the battery to the starter and the ignition switch. If the wiring is faulty, it will need to be repaired.
If the wiring is not faulty, check the brake system. The brake system is responsible for stopping the car. If the brake system is faulty, it could prevent the car from starting.

If you are not comfortable troubleshooting the problem yourself, you can take the car to a mechanic.
If you are troubleshooting the problem yourself, be sure to take safety precautions. Wear gloves and safety glasses, and disconnect the battery before starting any work.
If you have any questions, please feel free to ask.