The iPhone wants to update the iOS. You authorize it to do so. After completion, (it takes approx. 30 minutes), the phone will not boot up. All you get is a picture of iTunes and a plug indicating you need to plug it into the authorized computer you have always used.
This issue is caused by a corrupted download. The update for the iOS corrupted your current iOS rendering it unusable. The only option is to reset the iPhone to factory settings, with the new software installed. What happens to all your data?
When you plug the phone into your authorized computer, the iTunes software opens up with the option to Reset the iPhone. Before doing that, make sure you have a backup of your data so you can reinstall it after the reset is complete. To find that data, go to c:\users\(your name)\AppData\Roaming\Apple Computer\MobilSync\Backup. Look at the dates to the right of the folders. Look for the latest one and verify it is accessible. Open the folder. Although you will not be able to see any of the data, if you can see the files, then it is accessible. Note: if you are using Windows XP, navigate to C:\Documents and Settings\user\Application Data\Apple Computer\MobileSync\Backup.
Close out of the folder and proceed with the restore if you are satisfied with the date of the backup. Once complete, the iPhone will restart and reappear in iTunes. The option to continue will appear with two selections. Pick the one that will restore your data from a backup. It should already have your latest backup listed, (light gray lettering). If you want to use a different backup, click the down arrow and select the desired folder. Continue with the process.
When complete, you will need to reinstate your passcode, if you had one, and reinsert your iTunes account information, if you had an account. We then suggest opening your backup program and adding the backup folder you just used to the profile so your phone data is part of the backup plan.