Some users have reported problems with the iPad Pro freezing after charging and unable to respond to pressing the wake or home buttons.
Users are suggesting the culprit is iOS 9.1. Hopefully this is the case since a simple software update to iOS 9.2 can fix the issue.
Apple released a short support document explaining how to fix the issue. You need to force-restart the iPad Pro by holding the sleep/wake and home button together for 10 seconds.