The tags appended to your post suggests that you have a fifth generation iPad Pro 12.9”. This iPad Pro has the XDR screen with miniLED backlight and zone dimming - the backlight being actively controlled by system software.
Begin with a forced restart of your iPad - as this may clear an isolated software issue:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.
If the restart fails to resolve the problem, your iPad may have a screen backlight hardware issue. You may be well advised to seek assistance directly from from Apple Support.
Official Apple Support
Alternatively, if you have access to another Apple 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/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 test and assess your iPad. Here you will be in the best possible place to explore your available options.
Genius Bar Reservation and Apple Support Options - Apple