After you install the Windows 10 Technical Preview build 9879, a black or blank lock screen prevents you from logging on to Windows. When this problem occurs, the computer does not respond to keyboard or swipe actions.
This problem occurs because of a change in the behavior of WinLogon and LogonUI. This change introduces the potential for the deadlock.
An update is available to resolve this problem.
How to obtain and install this update
Windows UpdateUse Windows Update to automatically download and install the update.
PrerequisitesTo apply this update, you must have Windows 10 Technical Preview build 9879 installed.
Registry informationTo apply this update, you do not have to make any changes to the registry.
Restart requirementsYou must restart your computer after you install this update.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.