I had the same problem after I download the iOS 15. But I manage to fix it. After you unlock your iPad, I erased all the data with my iPhone in the app Find My. Since the iPad was still connected to the internet. Then the apple logo will Strat to load. Then I setup the iPad without a back up, why? because I tried and after setting up everything I got the same problem. So set your iPad from zero. Thats how I fix it.