Try connecting your iPad to iTunes and see if it will update from there. Make sure you plug your iPad directly into your computer’s USB port. Do not plug it into any attached keyboard or hub. Turn off your computer and your iPad. Then turn them both on again.
You might need to restore your iPad. Hopefully, you have a recent backup available via iCloud or iTunes. Additional tips are in this article (https://appletoolbox.com/2017/02/iphone-keeps-resetting-how-to-fix/)
Hope this helps!