You have to restore the device within iTunes. You want to use the same computer that you always sync with so that you can restore your app data and settings. You can restore with any other computer, but you will lose everything on the iPad.
Instructions can be found here.
You might need to restore using Recovery Mode. You can read about it here.
Recovery Mode Instructions
- Disconnect the USB cable from the iPad, but leave the other end of the cable connected to your computer's USB port.
- Turn off iPad: Press and hold the Sleep/Wake button for a few seconds until the red slider appears, then slide the slider. Wait for iPad to turn off.
- If you cannot turn off iPad using the slider, press and hold the Sleep/Wake and Home buttons at the same time. When the iPad turns off, release the Sleep/Wake and Home buttons.
- While pressing and holding the Home button, reconnect the USB cable to iPad. When you reconnect the USB cable, iPad should power on.
- Continue holding the Home button until you see the "Connect to iTunes" screen. When this screen appears you can release the Home button.
- If necessary, open iTunes. You should see the recovery mode alert that iTunes has detected an iPad in recovery mode.
- Use iTunes to restore iPad.