You need to place the device in recovery mode, and restore to factory settings:
Power down the device by using the sleep wake button and slide to power off.
Hold down the home button (and keep holding it) and plug the device in to the computer using iTunes.
When the itunes logo and usb cable picture display on the iPad, you can let go of the home button.
iTunes should prompt that it has detected an iPad in recovery mode, and it must be restored to factory settings before it can be used.
Follow the prompts.
Hope this helps.