Battery:
* Dead battery: This is the most likely culprit. A dead battery will power the lights (which require less power) but not have enough juice to crank the engine (which requires significantly more). Have the battery tested at an auto parts store. They can usually do this for free.
* Loose or corroded battery terminals: Check the battery terminals for corrosion (white, green, or blue powdery substance). Clean them with a wire brush and baking soda and water solution. Make sure the connections are tight.
* Faulty battery cables: Inspect the cables for damage or breaks.
Starter Motor:
* Faulty starter: The starter motor is responsible for turning the engine over. A bad starter might click when you turn the key, or make no noise at all. This requires professional diagnosis and replacement.
* Starter solenoid: This is a part of the starter system. A faulty solenoid can prevent the starter from engaging.
Alternator:
* Faulty alternator: While the alternator charges the battery while the engine runs, a failing alternator *might* still allow the lights to briefly work (using remaining battery power) but won't recharge the battery, leading to failure to start. A fully dead battery might be a symptom of a bad alternator. Have this tested as well.
Other Possibilities:
* Fuel system issues: No fuel, clogged fuel filter, fuel pump failure. You might hear the engine crank but not start if there's a fuel problem.
* Ignition system problems: Problems with the ignition switch, ignition coil, spark plugs, or distributor (if applicable) can prevent the engine from firing.
* Security system: Some cars have security systems that can prevent starting if the key isn't recognized.
* Immobiliser: This is a security device that will prevent the car from starting if it detects an unauthorised key.
* Low or empty fuel tank
What to do:
1. Check the battery: This is the easiest and most common fix. Test the battery voltage and look for corrosion.
2. Listen carefully: Does the engine crank (turn over)? If not, the problem is likely the battery or the starter. If it cranks but doesn't start, consider fuel or ignition issues. Any clicking noises are very indicative of a starter problem.
3. Call for help: If you're not comfortable checking these things yourself, call a roadside assistance service or a mechanic. They have the tools and expertise to diagnose the problem properly.
Don't attempt to jump start the car if you're unsure how to do it safely. Improper jump starting can cause damage to your car's electrical system.
