We have articles that can help you troubleshoot the device not appearing in iTunes, but it depends what operating system you're using - OS X or Windows. Below are both articles:
iOS: Device not recognized in iTunes for Windows
iOS: Device not recognized in iTunes for Mac OS X
To begin troubleshooting this issue on your iOS device, first follow the steps in the appropriate troubleshooting assistant below:
If the issue persists after following the troubleshooting assistant, continue with the in-depth steps in the appropriate article, testing to see if the issue is resolved after each section.
Hope that helps,