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Power button is necessary to put it to Recovery or DFU Mode to restore manually. You have now two options in here;
First is, you can do a erase from iCloud if it is connected to internet.
On your computer, go to www.icloud.com
Login to your iCloud/Apple account. Click Find iPhone. Select your iPad under All Devices. Click Erase button and it will be erased (if it is connected to WiFi network). This will erase it.
Second method is, you have to let the battery drain, then put it to recovery mode and do a restore. So first, go to this website and download the correct firmware for your device from here --> http://www.iclarified.com/8500/where-to-download-ipad-firmware-files-from
(The firmware is a big file, so download it and save it on desktop or any other folder).
Now let the battery drain. Yes, it may take a few days. You can constantly turn on the screen and play on iPad to make it drain faster. This will cause the device to turn off. Then download and install latest iTunes on your computer from here --> http://www.apple.com/itunes/download/
Now when it is off, connect it to your computer and open iTunes. Hold down the Home button for until it boot into the recovery screen which show iTunes and USB cable (you may need to hold down the Home button until it get a small charge and turn itself on to recovery mode). iTunes will detect it into recovery mode and ask you to restore it. Cancel it. Now press Shift key (or Option key if using Mac) and select the Restore button. This will ask you to select the firmware file you've downloaded above. Choose it.
Click Restore and this will restore it. Please let me know how it goes