What is the first action to take if an iPhone won't turn on?

When faced with an iPhone that won't turn on, the first action you should take is to charge the device. Often, a device may appear unresponsive due to a completely drained battery. By connecting the iPhone to a power source using the appropriate cable and charger, you give it the opportunity to recharge. It's important to allow it to charge for a while, as it may take a few minutes to show any signs of life if the battery is very low.

If charging does not resolve the issue, other actions, such as resetting the device or inspecting the hardware, can be explored subsequently. However, starting with charging is essential as it addresses the most common and simplest reason for an iPhone's failure to turn on.