iPhone 12 Will Not Turn On

First things first, be sure to check that your iPhone is not just discharged. Usually, the iPhone will give you a screen prompting you to plug it in when the battery is too low to power on, but if the battery is extremely low, there might not be enough energy to power the display. Plug your phone in for at least 10 minutes to ensure that your phone is not just discharged.

Tip: Be sure to check that your cable, charging block, and outlet are all functional before ruling out simple discharge as the culprit. You can try plugging in another iPhone to make sure that the setup is getting power if you suspect that one of these components may be dysfunctional.

If your iPhone stops powering on for no apparent reason, i.e. you didn’t drop it, expose it to water, or sustain other damage to it since it last turned on, the most likely culprit is a firmware crash. You’ll need to perform a forced restart to get your phone out of a bad firmware loop, which you can do by following these steps:

1. Press and immediately release the volume up button.
2. Press and immediately release the volume down button.
3. Press and hold the power button.
4. The apple logo should show up after around 10 seconds—once it does, release the power button.

Note that these button presses must be performed in quick succession. Additionally, if your phone does not turn on as expected, you should try charging the phone for at least 10 minutes before trying again.

Over time, the batteries in smartphones will start to degrade and hold less and less charge over time. If you’ve been using your iPhone 12 excessively and have accumulated multiple thousands of charge cycles, your battery may have degraded to a point where it can no longer hold a charge.

Additionally, lithium-ion batteries like the one in your iPhone 12 don’t like to be kept at the extreme ends of their charge range (fully-charged or fully-discharged) for long periods of time. If you’ve kept your iPhone 12 at one of these extremes (especially fully discharged) for multiple weeks or months, this may have degraded the battery life significantly, even if your charge cycle count is relatively low.

You can check for battery issues by seeing if your phone turns on while plugged in. If your phone does turn on while connected to a power source, you can check the battery health using the following steps:

1. Open the Settings app on your phone.
2. Navigate to the Battery settings tab.
3. Select the Battery Health option from the Battery menu.
4. Check the Maximum Capacity percentage.
   • If this number is below 80%, your battery has degraded significantly and you’ll notice a shorter life per charge, but you should be able to continue using your iPhone.
   • If the number is below 50%, your battery is due for a replacement, as your battery’s maximum power output may be too low to handle the performance of your phone.

If you find that your battery is what’s causing your issues, you can replace it by following this nifty iPhone 12 Battery Replacement Guide to replace it with a new battery.

The screen of the phone is responsible for displaying the user interface, and because it controls most of the input/output for the phone, a faulty screen may be mistaken for the phone not turning on.

If the phone was recently dropped and the screen is visibly damaged, a screen replacement may return the phone to working condition. However, you can test this by plugging the phone into an external display if possible. If you get an output on the external display, it means that your phone is still turning on, but the display is busted.

Another way to test for a damaged screen is to use a usb ammeter to confirm that the phone is charging, which confirms that the internals of the phone are still functional and points to an issue with the screen.

You can replace the screen by following this iPhone 12 Screen Replacement Guide to replace it with a new screen.

If none of the above solutions were able to turn your iPhone 12 back on, the issue may be with the main circuit board of the phone. The motherboard controls most of the phone's functionality, handling input sent in from the screen and buttons and processing it to produce the desired outcome. Also contained on the motherboard are the phone’s A14 system-on-a-chip (containing the CPU, GPU, and RAM in one unit) and flash memory (which houses all of the phone’s data storage), among several other chips.

Typically, the motherboard only goes bad when it experiences a significant impact, exposure to water, extreme temperatures, or when the BIOS settings are changed (like when a jailbreak attempt is unsuccessful). If you suspect any of the above have damaged your motherboard, you may need to replace it to regain functionality for your phone.