IPhone keeps looping on Apple logo after updating to iOS 6

I successfully installed IOS 6 on my iPhone 4 S. When done with verifying update, the iPhone shows the Apple logo on black screen and the status bar is running actively. But I accidentally turned the iPhone off and when it restarted, it just keeps on going back to the black screen with Apple logo.
It will disappear and then the iPhone will restart again and just keep on going back to the old screen. What can I do to resume the update?
