Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
The firmware of your iPad is corrupted during the update. Your have to force-restore the iPad to factory settings through iTunes by putting it to recovery mode, which will erase everything from it.
But if you've backed up your iPad to iTunes on your computer or iCloud, then you can restore all your data back after restoring your iPad from iTunes/iCloud. You can also redownload all your past purchases for free using the same account.
Follow the steps below to restore your iPad by putting it to recovery mode.
First download and install latest iTunes on your computer from here : https://www.apple.com/itunes/download/
Then connect the iPad to your computer with the USB Cable. Open iTunes and does iTunes recognize it ? If so, select it at the top left by clicking the iPad icon. This will bring Summary tab. Click Backup Now button and this will backup it. Make sure "To this computer" is selected.
Then on your iPad press and hold the Home button and the Power button at the same time for 10 seconds or until your iTunes shows a message telling you that it has detected an iPad in recovery mode.
Click Restore or Restore and Update button.
Note : Put it to recovery mode while connected to computer and also in recovery mode your iPad will show USB cable pointing to iTunes logo. And it will take a while to restore it.
Please let me know if that didn't work, so I will continue to help you until we fix the problem