Start here:
If you can't update or restore your iPad - Apple Support
If you continue to experience difficulties, you might be well advised to seek assistance directly from from Apple Support. You can contact the Support Team using the Get Support link at top-right of this page.
Alternatively, if you have access to another device, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to provide assistance:
Genius Bar Reservation and Apple Support Options - Apple