No this has nothing to do with rooted, you can download replacement home launchers from the market and these apps have a option to change the home buttons default options, the app Widget Locker also has a option for this also.. Any of thes apps installed could have changed the default...
When you press the home button what happens? This will help to know if a app has changed this setting..
Thank you Wildman for your replies.
You not being able to use the Home button is a huge inconvenience. When did this start? Was it after an application was downloaded? Please restart the device in Safe Mode and retest. This will help determine if it is being caused by an application. If it works in Safe Mode try deleting any new applications that were downloaded.
Was this the caused by liquid or physical damage on the device? Any other keys not working? If you have removed and reinserted the battery and retested. Please back up the device and Hard Reset