When your iPhone starts to go dead, it warns you. When 20 percent of the battery life remains, you get a pop-up message, and the battery life meter on your home screen turns red. If you don't heed the phone's warnings and charge it up quickly, you may end up with a dead iPhone on your hands. Starting it back up once it's dead takes a little more than just plugging it in.
Plug your iPhone into an outlet, computer or vehicle to begin charging. You'll need a USB-to-dock connector cable to plug it into a computer, the cable and a USB power adapter to plug it into an outlet, or an iPhone car charger to plug it into your car.
Wait for the low battery screen to appear, which is a picture of a nearly empty battery with a lightening bolt underneath it. The phone may stay off for two to three minutes before it powers up, but when it does this screen will appear.
Wait at least 10 minutes after the low battery screen appears. Press the "Home" button at the bottom of your iPhone and slide the button across the bottom of the screen to unlock it.