Apple has two articles to cover this - one for iTunes on Windows OS and the other for iTunes on Mac OS:

- https://support.apple.com/HT204095#PC - iPhone, iPad, or iPod not recognized in iTunes for Windows PC
- https://support.apple.com/HT204095#Mac - iPhone, iPad, or iPod not recognized in iTunes for Mac

Both start with If your iPhone, iPad or iPod won't connect to iTunes on your PC, try the steps below. Just follow the guide for your OS and consider [asking a follow-on question][1] if you get stuck with a specific step. Be sure to list the versions of OS and iTunes in that post as those details will help us help you.


  [1]: https://apple.stackexchange.com/questions/ask