If you don't already have an established iCloud or iTunes backup, it may now be too late. If your files and data are important to you, always ensure that you have a backup - before disaster strikes.
Given your description, there are clearly steps that you have yet to try - perhaps the most appropriate being a factory restore. You should note that this step will completely reset and erase the iPad - and install a fresh copy of the most recent version of iPadOS supported by your iPad:
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Otherwise, you would be best advised to seek assistance directly from Apple Support. As already outlined, 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 Apple Support App:
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 provide assistance:
Genius Bar Reservation and Apple Support Options - Apple