Thank you guys, my Ipad have comebacked when I tried to press Home and power buttons for 1 minute .. I will observe it, if the problem is again, I will return it.
The exact same thing happened to me is morning. Battery was 50% and this morning I can't start the thing.
I spend 50 hours working PDF's since I have this thing and now all is lost ?? I know make backups but wtf I have te iPad Air 2 for 2 weeks now, you don't expect a perfectly fine working unit to not just start in the morning after half a month. The funny thing is I read this thread just yesterday and thought to myself better make an update tomorrow. Very nice.best timing.
Frustrated right now. I will report back if resetting might help like OP has suggested......
Edit : sorry guys I panicked . All data is still here and the device works fine despite the problem. I thought it would be a factory reset. The trick worked, but it is interesting I experienced th same thing.
OP , did you switch to flight mode before locking The device ? Maybe there is a related iOS 8 bug