All iPhone models store your personal data, app settings, and work documents in non-volatile memory. If you turn off the phone, if the battery runs out, or if the phone freezes and you have to restart, you won't lose any data other than the unsaved files you had open at the time. However, if you have to completely restore your phone to factory settings, you will lose all your data up to the point of your last iTunes or iCloud backup.
Normal restart
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A normal restart of your phone (what Apple calls a reboot) won't cause you to lose any data, except for unsaved files you opened in apps without auto-saving. To restart the phone, press and hold the "Sleep/Wake" button until a red slider appears on the screen. Slide the bar across the screen to close the phone. After the screen turns off, press and hold the "Sleep/Wake" button again until the device boots up.
See AlsoThe history of Apple iPhones
Forced restart
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If your phone freezes and the power off slider doesn't show up, you can reset it. This is a forced restart. This causes you to lose unsaved data in running apps, even though those apps normally save automatically when closed. To reset, hold the "Sleep/Wake" button and the "Home" button at the same time for about 10 seconds. The phone will turn off and then restart automatically.
to recover
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If the phone continues to have issues after a reset, you can restore it to factory settings. This process will erase all data on the phone, including your apps, your app data, your contacts, and your calendar, and reset it to factory settings. To restore an iPhone, connect it to your computer, open iTunes, select iPhone and press the "Restore iPhone" button on the device's "Summary" tab. Tap "Restore" to confirm and start the recovery. If you backed up before restoring the backup, you can restore your lost data after the phone restarts by tapping "Restore Backup" in iTunes or "Restore from iCloud Backup" up" on the phone depending on how you backed up your device.
Making backup copies
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Backing up your iPhone regularly is the only way to protect your data if you need to restore the phone. Every time you sync your iPhone with iTunes on your computer, a backup is created automatically. Alternatively, you can enable iCloud backups in the "Settings" app by tapping "iCloud" then "Storage & Backup" and turning "iCloud Backup" to "On". After you set this option, the phone will automatically back up when it is connected to power and Wi-Fi and not in use.
