iPad Air 2 won't fully reboot after installing iOS 15
I downloaded and installed iOS 15 to my iPad Air 2. It appears the install worked correctly, the iPad rebooted several times during the installation (normal). I used it successfully the next day, but after I shut the iPad off at the end of the day, it would not fully start up again the following day. It would start booting-up, but never got past the point where the Apple logo appears on the screen. I tried forcing a reboot multiple times by holding down the power and home buttons simultaneously till the iPad turned off and started to reboot, but it never fully rebooted.
What is likely to be wrong, and what should I try next?
iPad Air 2 Wi-Fi, Cellular