1. Press and hold both buttons 'sleep / wake' and the 'home' at the same time at least 10 seconds until the Apple logo appears on the screen.
2. If it does not work, delete all existing content on the iPhone. From the home screen, select Settings>> General>> Reset> Erase All Content and Settings.
If it still does not work, restore the iPhone software:
1. Make sure you have an internet connection and no installation of latest version of iTunes from www.apple.com / iTunes
2. Connect the iPhone to the computer,
3. In iTunes, select iPhone in the source pane and click the summary tab,
4. Click 'Check for update' iTunes will inform you about the iPhone needs to be updated or not,
5. Click Update to install the latest version or restore to restore to original settings and erase all existing data on the iPhone. Follow the instructions on the screen for further instructions
Other information about the reference iPhone:
* IPhone help: Tap Safari> Bookmarks icon>> iPhone user guide,
* IPhone details: www.apple.com / iPhone
* IPhone support and the iPhone user guide: www.apple.com / support / iphone