Syncing an iPhone with iTunes lets you transfer apps, as well as audio and video content, from the computer onto the phone. You may consider reformatting the iPhone's hard drive if the phone is no longer syncing with the iTunes software. Erasing all of the phone's memory should fix the problem, but doing so may not be necessary. The fixes listed below could allow you to once again sync the iPhone with iTunes without losing any of the phone's data.
Connect the iPhone to the computer with different Apple USB cords. These cords are notorious for wearing out easily, and wear could prevent the iPhone from syncing with iTunes.
Attach the iPhone to different 2.0 USB ports on the computer. A faulty USB port could keep the computer from recognising or reading the iPhone.
Charge the iPhone for 30 minutes via an iPhone charger or a computer. Attempt to sync the phone after 30 minutes.
Hold down the iPhone's power/sleep button until the red slider shows up on the screen. Slide the red slider from left to right, and the iPhone shuts down. Hold the power/sleep button again until the device turns on. Then, try to sync the iPhone.
Restart the computer. Doing so also restarts the iTunes software. Attempt to sync the iPhone after the computer fully reboots.
Visit the "Download" page on the Apple iTunes website. Download/re-download and install the current version of iTunes.
Reset the iPhone. Hold down both the iPhone's "Home" and power/sleep buttons for no less than 10 seconds. The iPhone powers down. Turn the iPhone back on, and try to sync the device.
Take the iPhone to an Apple or AT&T Store if none of the above fixes allows you to sync the iPhone.