Next, let's make sure the problem isn't with the USB port by plugging the iPad into a different port than the one you previously used. If you are using a USB hub or plugging it into an external device like a keyboard, make sure you use a USB port on the computer itself.
If plugging the iPad into a different USB port solves the problem, you may have a bad port. You can verify this by plugging another device into the original port.
Most computers have enough USB ports that a single broken one is not a big deal, but if you find yourself running low, you can purchase a USB hub at your local electronics store.