Same problem here.
Last Windows 10 build, when I woke up today my phone was turned off. When I try to login it appears Nokia, the Windows 10 logo for a sec, the gears and then the sad face, then it turns off.
Lumia 930.
I can't do the soft or the hard reset, it keeps looping. I am trying the use Windows Phone Recovery Tools, but it isn't working.
Edit: After some tries the Windows Phone Recovery Tool recognized my phone and it is working now (Windows 8.1). My phone was encrypited.