Why does my Ipod not appear on my itunes?

When i plug the usb cord from my ipod to my itunes, my itunes prgram pops up and says " This Ipod cannot be used because the Apple Mobile Device service is not started." I don't know hwat they are trying to say. The ipod is suppose to sync auto, right guys? this is so messed up. Can you guys tell me how to fix it again? without installing itunes and installing it again. Thanks.