If a car makes a clicking sound but won't turn over when the key is turned, it indicates a problem with the battery or battery cables, the starter solenoid or the starter motor. The problem is almost always a dead battery.
Samarins.com suggests that if the engine clicks but won't start when the key is turned, an attempt should be made to jump the car.
Jumper cables should be held so none of the clamps touch each other. The assisting car should be nearby with the ignition off. Connect a red cable first to the positive terminal on the dead car, then connect the other red cable to the positive terminal on the assisting vehicle. Next, connect the associated black cable to the negative terminal on the assisting vehicle, and finally connect the last black cable to a grounded metal part on the dead car. Start the assisting vehicle, wait a few minutes, then attempt to start the dead car. If the dead car starts, wait a few minutes, then disconnect the jumper cables in the reverse order they were connected.
If the dead car still won't start, or fails to start the next time it is used, a mechanic should be consulted.