You can find troubleshooting guidance for FaceID and the associated TrueDepth camera here:
https://support.apple.com/118243
If you are unable to resolve the problem yourself, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively 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/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 hands-on test and assess your iPad. If a hardware fault is identified, you'll be in the best possible place to explore your available options:
Genius Bar Reservation and Apple Support Options - Apple