Make sure that you have the latest version of iTunes. iPad requires USB 2.0 to function properly. Unplug your iPad from your computer, and plug it into a different USB port that is built into the computer.
Turn your iPad off and on.
Disconnect your iPad from your computer, then turn your computer off and on and re-connect.
Especially for Windows users, if the above steps do not resolve your issue, completely uninstall iTunes and then reinstall the latest version of iTunes.