Your iPhone could be dead. The hardware is still working but the software is done. All that is remaining is for you to clear it and begin all over again. If you are lucky, you will be backed up, or at least have your songs and images saved somewhere else, since putting your device in DFU (Device Firmware Update) mode start everything afresh.
When will this help? When your iPhone's OS has totally cracked and is not stable to function. DFU will let you load a completely factory-fresh version of the OS, and can also restore your iPhone to working order.
This is like the final potential solution. It is a delicate process and can also be tasking. If the buttons are held for too long or too short, or they are held down and released in the wrong order, it will not work and you will need to begin all over again.
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How Can I Put An iPhone In DFU Mode?
On an iPhone 7 and earlier:
- If you are an iPhone 7 user, power your iPhone on. If you are using an earlier model, disable your iPhone.
- Plug your iPhone into your PC.
- Ensure iTunes is running on your PC.
- Make sure the On/Off button (located on the side) and Volume Down Button are held down for above ten seconds. If you are using an iPhone 6 or earlier, the On/Off button is the lock screen button on top, and you will press Home rather than Volume Down.
- Release the On/Off Button. If the Apple logo is visible, you held On/Off too long and will have to begin all over again.
- Continue holding Volume Down (or Home) until iTunes on your PC says, "iTunes has detected an iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes." If the display on your iOS device is not black, you will have to try it once more.
- Tap OK.
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On An iPhone 8 And Newer
- Get your iPhone plugged into your PC.
- Confirm that iTunes is running on your PC.
- Press and release the Volume Up button.
- Press and release the Volume Down button.
- Make sure the On/Off button is held on until the display turns black.
- Make sure the Volume Down button is held down as you keep holding the On/Off button for 5 seconds.
- After 5 seconds, release On/Off as you keep holding Volume Down. If the Apple logo is visible, On/Off was held for too long and you will have to start all over again.
- Continue holding Volume Down until iTunes on your PC detects an iPhone in recovery mode and prompts to have it restored. If the display on your iPhone is not black, you have to try again.
- Tap OK.
Be aware that if your PC reveals any other thing but the above, or your iPhone screen displays any visuals but black, you are not in DFU mode.
But, you might have mistakenly triggered Recovery Mode, which will let you try and just update your software.
If it is recovery mode, iTunes will prompt you to update or restore your iOS device.