My iPhone Won't turn on: How to Fix a Dead Device
You've just pulled the iPhone out of your pocket and it won't turn on when you press the Power button. You just got a black screen.
A dead iPhone is a scary thought, but don't freak out yet. You can probably make it boot again. Here is what to do if your iPhone 6/5s/5/4s/4 won't turn on.
iPhone may fail to turn on if its battery is completely dead. Plug in your device and let it charge for a while – give it fifteen minutes, perhaps. No matter how worried you are, you can’t just plug it in and expect it to respond immediately. Give it a few minutes to charge and then try turning the iPhone on as usual while the device is still connected to a power source.
Forcibly reboot iPhone by holding down Power and Home button at the same time till the Apple logo appears. It will take 10- 15 seconds.
The force reboot solution works if your iPhone is crashed or frozen, which can sometimes cause an unresponsive iPhone with a black screen that appears dead because it isn’t responsive to anything.
If your iPhone gets the Silver Logo but restarts there, again and again, you can fix this easily by entering into DFU Mode. Once you are in DFU mode you can downgrade your iPhone iOS in case the new version has created the problem.
When the screen remains black and won't turn on, you may need to restore your iPhone to factory settings. This can be done by syncing your iPhone with iTunes and then click Restore.
Please note that this option will erase all the data and settings on your phone, so make sure you've backed up iPhone before restoration.
The proximity sensor is a small part of iPhone which helps to lock the iPhone screen when it is near your head/ear/face. Though it is very rare, a malfunction in this sensor would simply mean that your phone will be locked even if your face has moved away from it.
To fix this problem, just reset your iPhone settings: Tap the Settings > General> Reset>Reset All Settings.
If nothing here worked, your iPhone likely has a hardware problem. If it's still under warranty, take it to the nearest Apple Store (or just contact Apple) and ask some of the geniuses what is wrong with it. They should be able to give you a proper diagnosis. Even if your device is out of warranty, you can potentially get the fix from Apple by paying.