OK i had a similar problem last Tuesday 26/07/16, i wanted to ask Cortana on my Phone (lower spec phone than the 950 a Lumia 640 xl) to redmine when i get Home to get Peter's case and it was a embarrassing moment.
Embarrassing cause many were listing and i was driving the car and like F**** Cortana you let me down.
Just do a soft reset ie: Press down volume and start keys till the phone vibrates, it's just a Microsoft Account error
So True, We Get So familiar with high tech devices, Computers hibernating and then woken up, not really a re-boot.
I work in IT and personal computers, 1st task i do is re-boot , works wonders 90% of the time :grin: