There are a number of reasons why iTunes may not successfully recognize an iPhone. Apple has provided a series of steps for both Mac OS X and Windows users for troubleshooting these types of issues.
For Mac OS X users, Apple recommends checking several items to resolve an iPhone not recognized by iTunes. First, a user should check that the Mac meets the minimum system requirements. Then, a user should download the latest updates available for the Mac if they are available; the Notifications pop-up on the Mac specifies if updates are available for download. After this, the user should check the USB connections and ensure that the computer uses at least USB 2.0. Next, the user should disconnect other USB devices, connect the iPhone to a USB port on the Mac, and restart the computer.
For Windows PC's, Apple claims the most common issues are that either the device cannot be restored in iTunes; a certain special character is next to the device's entry in Device Manager; or an update or restore caused the device to no longer be recognized. Apple recommends first updating iTunes, checking the USB cable and ensuring that an Apple-compliant USB data transmission cable is being used. If the issue persists after this, the user should then verify that Apple Mobile Device Support is installed. The program appears in the Programs list in the Control Panel if it is installed. The user should then restart the Apple Mobile Device Service using the Services area of the Control Panel. If the issue persists after this, the next step is to verify that the Apple Mobile Device Driver is installed in Device Manager; it appears under the Universal Serial Bus Controllers section if installed. Finally, if the issue is still there, a third-party piece of software from other vendors may need to be removed for iTunes to work correctly.