NoGoodNick - September 22, 2012 at 6:52pm
Once you get jammed up on an update, it's usually a pain to get it started again. If you've already done a complete restore (meaning you lose all your data) and it still doesn't work, then you'll have to try more drastic actions.
Before you do, though, try shutting it down and trying again, making sure you have a reliable network connection (home network) and plenty of charge to last for the update. You'll also need at least 2gb of free space on your device.
If that doesn't work, then download iOS 6 to your computer, then force it to upgrade from an existing copy (hold when clicking on "Upgrade"), forcing it to accept another source. Hopefully that'll work.