Erasing all content and settings (hard reset) may help with a blank or frozen screen, apps crashing or freezing, keypad/touchscreen problems, can't hear, device making noises, can't make or receive calls, and device won't sync.
Erase all content and settings only if absolutely necessary as it removes all data including:
An alternate method (restore via iTunes) is available if the device is unresponsive or won't power on.
For specific error messages related to iTunes update and restore, refer to iOS update and restore errors. Advanced iTunes troubleshooting and configuration should be handled by Apple (scroll down to "Product and Technical Support" section).
From the Home screen, tap Settings > General > Reset.
Tap Erase All Content and Settings.
If presented, enter your Restrictions passcode.
Tap Erase iPhone.
Allow several minutes for the reset and reboot processes to complete.
To set up the device after the reset has completed, refer to Initial Activation and Setup.