If your iPhone 15 is completely black and won't respond, the first thing to try is a force restart. This is different from a normal restart and can clear out temporary software glitches that freeze the phone. Press and quickly release the Volume Up button, then press and quickly release the Volume Down button. Finally, press and hold the Side button until you see the Apple logo appear on the screen.
Keep holding the Side button until the Apple logo appears, which might take longer than 10 seconds. Release it when the logo appears. If your iPhone still won't turn on, charge it for one hour and then try again.
Check Your Charger and Cable
Your iPhone 15 might simply have a dead battery. This model was Apple's first to switch from Lightning to USB-C, so it's crucial you're using a USB-C cable. Plug it into a wall outlet using an Apple USB-C power adapter or a reputable third-party one that supports USB Power Delivery.
Let it charge for one hour, then try turning it on again. Check the USB-C cable and power adapter for damage, and don't use damaged accessories. Try a different cable, power adapter or wall outlet. Make sure all connections are firm, and remove the case if it prevents the cable from seating fully.
Look for Signs of Life
While it's plugged in, press the Side button once. Look closely at the screen for any sign of activity or a low-charge battery icon. If you see the battery icon but it won't progress, the charging port might be dirty.
Use a dry, soft-bristled brush to gently dislodge any lint from the USB-C port. Never use a metal object like a paperclip. A blocked port is a common reason an iPhone 15 won't charge or turn on.
Understand the 80% Limit Feature
The iPhone 15 has a charge limit that can be set from 80% to 100% in 5% steps. A limit below 100% stops charging before the battery is full. If your phone stops charging at the selected limit, change the limit if you want it to charge further.
Once your phone is on, you can check this in Settings > Battery > Charging. If the phone is off, leave it connected to power for one hour and then try turning it on again.
Try a Computer Connection
If a wall outlet isn't working, connect your iPhone 15 to a computer's USB-C port using your cable. Make sure the computer is powered on and not in sleep mode. A powered USB port on a Mac or Windows PC is another charging option. Check that the cable is firmly connected, and try another USB port if needed.
Check for your iPhone in the sidebar of Finder on a Mac or the Apple Devices app on a Windows PC. If the computer recognizes it but the screen stays black, try the force restart again. If it still won't turn on after charging, it may need service.
Check for Overheating
The iPhone 15 can get warm during fast charging, especially with MagSafe or certain cases. If the battery gets too warm, charging above 80% may be limited until it cools down. Unplug the phone and remove it from any wireless charger.
Move the iPhone and charger to a cooler, shaded spot and let the phone cool before charging again. Then, try charging it again without a case on. If it still won't charge, check the cable and adapter for damage and try a different cable or charger.
Use Recovery Mode
If your iPhone stays on the Apple logo for several minutes without a progress bar, shows the 'Connect to computer' screen or isn't recognized by your computer, try recovery mode. Update your Mac or install the latest Apple Devices app on Windows, then open Finder or the Apple Devices app.
Connect your iPhone 15 to the computer with a USB-C cable. Quickly press and release the Volume Up button, then the Volume Down button. Then, press and hold the Side button. Keep holding it even after the Apple logo appears, until you see the "Connect to computer" screen on your iPhone (a cable pointing to a computer icon).
Your computer will then give you the option to Update or Restore. Choose Update first, as it will reinstall iOS without erasing your data. This can fix deeper software corruption preventing a normal boot.
Consider a DFU Restore
DFU (Device Firmware Update) mode is a deeper restore than recovery mode. If recovery mode cannot update or restore your iPhone, it may need service. The button sequence is very specific and timing is key.
With the phone connected to the computer, press and release Volume Up, then Volume Down. Now, press and hold the Side button for 10 seconds. After 10 seconds, without letting go of the Side button, also press and hold the Volume Down button for 5 seconds. Then, release the Side button but keep holding the Volume Down button for another 10 seconds.
If the screen stays completely black, the computer should detect a device in recovery mode. You can then initiate a restore through Finder on a Mac or the Apple Devices app on Windows, which will erase all data and install a fresh copy of iOS.