If you are experiencing problems with an iPhone that will not turn on, you may need to restore the device to its original settings. Apple offers a recovery mode setting to troubleshoot iPhone issues that have can't be resolved otherwise. You may be able to recover your iPhone to its original settings by connecting the phone to your computer and opening iTunes.
Troubleshoot
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Ensure your iPhone is charged by plugging it in for at least 10 minutes. If the home screen or low battery icon appears, keep the phone plugged in for a full charge to see if the issue is resolved. If nothing appears after 10 minutes, reset your iPhone by pressing the sleep/wake and home buttons together for at least 10 seconds until the Apple logo appears. If no logo appears, you may need to restore your iPhone through recovery mode in iTunes.
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Restore
Connect the iPhone to your computer and open iTunes. If iTunes is able to recognize the iPhone, you can navigate to the "Restore" option in the iPhone's information settings in iTunes to restore the phone to its original settings.
Recovery Mode
If after troubleshooting and restoring the issue is still not resolved, you can try entering the phone into recovery mode. To enter recovery mode, disconnect the iPhone from your computer and power it off by holding the sleep/wake button until the slider appears to power the phone off. If the phone will not turn off, press and hold the sleep/wake and home buttons at the same time and release when the screen is black. Once the device is off, press and hold the home button and plug the phone into your computer. Continue to hold until the "Connect to iTunes" screen appears. While iTunes is open, the recovery mode alert will appear and you can now use iTunes to restore the device.
Data
Restoring an iPhone will erase all user content on the device, but any previously saved backup can be restored through iTunes.