If your vehicle is showing nothing happens when turning the key (no crank, no click), here are the most likely causes and the parts you'll need to fix each one.
The battery has been drained completely, lacking even the minimal voltage required to trigger the starter relay or solenoid. This is often caused by leaving lights on, parasitic draw, or an old battery reaching the end of its life.
Corrosion buildup or loose terminal clamps prevent electrical current from flowing from the battery to the starter system. This is a very common installation error after a recent battery replacement if the clamps were not tightened properly.
The starter motor or its attached solenoid has suffered an internal electrical failure (like worn brushes or a burnt coil). When this happens, it cannot engage or make a sound even when receiving full power.
The vehicle's computer relies on a switch to verify the transmission is in Park/Neutral or the clutch pedal is fully depressed. If this switch fails, the car intentionally disables the starter to prevent accidental lurching.
The electrical relay that acts as a bridge to send high-amperage power to the starter has failed, or a main ignition fuse has blown, completely cutting off the starting circuit.
The electrical switch located behind the key cylinder is worn out. When you turn the key, the physical motion is no longer translating into the electrical 'start' signal.
The vehicle's anti-theft system does not recognize the transponder chip in the key, or the key fob battery is dead. The computer responds by completely disabling the starting system.