Try a Reset: Hold the Sleep and Home button down for about 10 to 15 seconds to go into Recovery Mode.
If the Reset doesn't work, try and force iPad into Recovery Mode:
1. Turn off iPad
2. Connect USB cable to computer; leave the other end alone
3. Press and hold the Home button down and connect the docking end of cable to iPad
4. Continue holding the Home button until you see the "Connect To iTune" screen
5. Release the Home button
6. Open iTune
7. You should see "iTunes has detected an iPad in recovery mode"
8. Use iTune to restore iPad
Note: You need to be patient and repeat the above many times to recover your iPad
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