After an Update
If you selected the option to update your iPhone or iPod touch or restored from a backup:
iOS 3.0 added a new application for the original iPhone, iPhone 3G, and your iPod touch (2nd generation) called Voice Memos. If you already had 16 applications on your Home screen (not including the four applications in the bottom row), or if you had moved the Text application (for SMS messages), applications may be moved to a new second screen after the update or restore. In this case, applications that were previously on your second screen will now be on the third screen.
If you move the application or applications on the second screen to the third screen, the third screen will move back to being the second screen. (In other words, if you move all the applications from the new second screen, your iPhone or iPod touch will remove the now-empty second screen, so that the third screen will become the second.)
After a Restore
If you selected the option to restore and did not load your applications or settings from a backup:
If you restored your iPhone to install an iOS update, you will need to sync your applications from the computer as explained in this article. If you purchased the applications on the iPhone and do not have them on the computer, you may redownload them following the steps in this article.