How do I get the unlock screen to appear after software update?

I have been using my Ipod Touch 4gen for nearly three years under several versions of the operating system, currently IOS6. The system recently kept telling me that I needed to upgrade to the latest version of IOS6 and even downloaded the changes and told me to install them. I started the install and left the device for an hour or so. When I came back there was no indication of whether the install had completed or not. When I press the Home button, I do not get the Unlock screen, only an empty white (or grey) screen. How should I proceed?