Force your iPad into Recovery Mode.
1. Disconnect the USB cable from the device, but leave the other end of the cable connected to your computer's USB port.
2. Turn off the device: Press and hold the Sleep/Wake button for a few seconds until the red slider appears, then slide the slider. Wait for the device to turn off.
3. While pressing and holding the Home button, reconnect the USB cable to the device. The device should turn on.
4. Continue holding the Home button until you see the "Connect to iTunes" screen. When this screen appears, release the Home button.
5. iTunes should alert you that it has detected a device in recovery mode. Click OK, and then click Restore to restore the device.
Note: You need to be patient and repeat the above many times to recover your iPad. Data will be lost.