Charge it properly for an hour
A deeply discharged battery can take fifteen to thirty minutes on a charger before it shows any sign of life, and a phone that has been flat for weeks can take longer. Use a wall adapter rather than a computer port or a car charger, and leave it alone for an hour.
If a charging screen appears at any point, this was the answer. That is genuinely the most common cause of a phone that seems dead.
Rule out the cable and adapter
A cable that fails to carry power is common and invisible. Try a different cable and a different adapter, and check the phone's charging port for lint — compacted pocket lint is a frequent and completely fixable cause. A wooden toothpick, gently, is the safe tool.
Wireless charging is a useful test on models that support it: if the phone charges wirelessly but not by cable, the port or cable is the fault rather than the battery.
Force restart
A phone that is on but frozen with a black screen looks exactly like a phone that is off. Force restart it: volume up, volume down, then hold the side button for about ten seconds until the Apple logo appears.
Do this while it is connected to power, and hold longer than feels necessary. Our force restart guide covers older models, which use different sequences.
If it shows the logo and then dies
A phone that reaches the Apple logo and shuts down, or loops there, has a software or battery problem rather than a power one. If it only stays on while plugged in, the battery has failed — our replacement guide covers that.
If it loops regardless of power, our Apple logo guide works through recovery mode, which can reinstall iOS without erasing your data.
Try a computer
Connect the iPhone to a computer and see whether Finder or the Apple Devices app detects it. A device that appears there is alive even with a black screen, and can be restored — try recovery mode, and DFU mode if that fails.
A phone that a computer cannot see at all, on a known-good cable, is pointing at hardware.
When it is hardware
Liquid exposure, a drop, or the phone getting extremely hot before it died all point to physical damage, and no amount of restoring fixes those. Nor does a phone that fails a DFU restore.
Back up nothing, change nothing further, and take it in. Continued charging attempts on a liquid-damaged phone can make the damage worse rather than better.
Frequently Asked Questions
Why won't my iPhone turn on?
Most often a deeply discharged battery, a faulty cable, or a frozen system. Charge it for an hour on a wall adapter, then force restart.
How long should I charge a dead iPhone?
At least an hour on a wall adapter before concluding anything. A phone flat for weeks can take longer to show a charging screen.
My iPhone only works when plugged in — what does that mean?
The battery has failed. A replacement restores it and costs far less than a new phone.
What if a computer cannot detect my iPhone at all?
On a known-good cable, that points to hardware damage — liquid, a drop or a logic board fault — and needs service.