If your iPhone fails to boot up repeatedly you may have to restore it with iTunes and your computer. To restore the iPhone completely follow these steps:
1. Before you connect your iPhone with the USB cable, start iTunes. Make sure iTunes is finished downloading podcasts, updating, etc.
2. Connect your iPhone to the computer and wait 10-15 seconds for the connection.
3. Press and hold both the sleep/wake button and home button on your iPhone.
4. After 10 seconds, release the sleep/wake button and continue to hold the home button only.