If your car's engine isn't turning over but the lights come on, there could be several reasons for this issue:

1. **Dead Battery**: A common reason for this problem is a dead or weak battery. Even though the lights are working, the battery may not have enough power to turn the engine over. This can happen due to leaving accessories on while the engine is off, a faulty charging system, or an old battery that needs replacing.

2. **Starter Motor Issues**: The starter motor is responsible for turning the engine over. If it's faulty or failing, it may not engage properly, preventing the engine from starting. This can be due to worn-out components, electrical issues, or a faulty solenoid.

3. **Ignition Switch Problems**: The ignition switch sends power to the starter motor while starting the engine. If the switch is faulty, it may not transmit the necessary electrical signals to start the engine, even if the lights and other accessories are working.

4. **Faulty Starter Relay**: The starter relay is a small part that controls the flow of electricity to the starter motor. If it's defective, it can prevent the starter motor from engaging, resulting in a no-start condition.

5. **Faulty Fuel System**: If the fuel system isn't delivering the correct amount of fuel to the engine, it may not start. Issues such as a clogged fuel filter, fuel pump failure, or fuel pressure regulator problems can lead to starting issues.

6. **Ignition System Problems**: Problems with the ignition system, such as a faulty ignition coil, distributor, or spark plugs, can also prevent the engine from starting, even if the lights come on.

To determine the exact cause of the starting issue, you may need to perform some troubleshooting or have a professional diagnose the problem if you're not familiar with car mechanics.