This support page would be an excellent starting point:
If you can’t turn on or sign in to iMessage or FaceTime - Apple Support
If the problem persists, you might be well advised to seek assistance directly from from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044