If you run bitlocker and have it set to prompt for a password and find that the system powers of before you can enter the password then do the following.

From an elevated (admin) command prompt


bcdedit /set {bootmgr} bootshutdowndisabled 1

Should prevent it from happening again… although may need to be re-applied after windows 10 installs a new build.

Thanks to stateofloveandtrust for posting the solution on the Technet forum.  https://social.technet.microsoft.com/Forums/windows/en-US/932630c2-ae3d-4cbd-8d79-a492806363ea/windows-81-bitlocker-automatic-shutdown-during-password-prompt?forum=w8itproinstall