any one or a combo may help
Try a different USB port
Try a different USB cable
Give your iPad a reset. Hold down the sleep and home keys for about 20 seconds. When you see the silver apple, let go and let it reboot and try again.
Restart your computer
Make sure your iTunes is up to date
If you go through all that and you stillcan't get iTunes or your computer to see your iPad, go into the control panel, programs, add/remove programs. Uninstall Apple Mobile Device Service, Apple application support. THen redownload and reinstall iTunes and try again.
iPad not appearing in iTunes
iOS: Device not recognized in iTunes for Mac OS X
iOS: Device not recognized in iTunes for Windows
iTunes for Windows: iTunes can’t contact the iPhone, iPad, or iPod software update server
iTunes for Windows: Device Sync Tests
IOS: Syncing with iTunes
Apple - Support - iPad - Syncing
iTunes 10.5 and later: Troubleshooting iTunes Wi-Fi Syncing
iOS: “Not enough free space” alert when trying to sync
You may need to delete iTunes on your computer and then reinstall.
How To Completely Uninstall and Remove All Traces of iTunes