An iPhone may stop functioning normally for several reasons. It may have run out of battery power, or a corrupted download may have caused it suddenly to freeze and ignore your screen taps. An iPhone might even disable itself deliberately for security reasons. If you repeatedly enter a wrong passcode, for instance, a red "iPhone is disabled" message screen will appear and block you from using the device further. Thankfully, you can take steps to methodically rule out the reasons why your iPhone is malfunctioning and restore it with a hard reset or full restoration.
Recharge your iPhone for at least 10 minutes by connecting it to a power outlet using the cable and power adaptor that came with the phone. If your iPhone's problem is a lack of power, then within a couple of minutes it will display the image of a nearly empty battery containing a red sliver of energy. Consult your nearest Apple retailer if the battery's red portion flashes three times and the screen goes black; this may indicate that your device is not receiving power due to a faulty battery or charging element.
Turn off your iPhone by pressing the Sleep/Wake button on top of your device and then sliding the red slider to the right to power off. Next, turn your iPhone on by pressing the Sleep/Wake button until the Apple logo appears. If your iPhone does not restart, simultaneously press and hold the Sleep/Wake button and the circular Home button on the bottom-centre on your device. Do this for at least 10 seconds until the Apple logo appears.
Restore your iPhone if the hard reset described in Step 2 did not work, or if your repeated entry of wrong passcodes has blocked you from using your iPhone with a red "Your iPhone is disabled" message. Connect your iPhone to a USB port on the computer or laptop on which you last synced your iPhone. Launch iTunes and update it to the latest version as needed.
Click your iPhone under the Devices list on the iTunes sidebar to the left. Next, select the Summary tab to the right and click the Restore button. Because this step will reset your iPhone to its original factory condition and delete all data and media from your device, you first may want to back up your iTunes library, playlists, apps, books and App Store purchases.
Click "Back Up" when asked to confirm your intentions in the next pop-up screen. Next, select "Restore" and wait for your iPhone to restart itself. Keep your device connected to your computer as your iPhone cycles through the Apple logo to a "Connect to iTunes" screen and finally to a "iPhone is activated" message.
Click the backup you want to restore on your iPhone from the "Set Up Your iPhone" screen in iTunes. Then click the "Continue" button to complete restoration.
When connecting your iPhone to your computer or laptop, always plug into a USB port directly on your computer or laptop, not into the low-power USB ports of a keyboard or other external USB hub. If you have just recently backed up your iPhone, choose "Don't Back Up" in Step 5. If you encounter problems while restoring your iPhone, you will need to place your device in recovery mode before attempting a second restoration.