The lock screen in Windows 10 Technical Preview Build 10061 might appear as a black screen with a mouse cursor. When you sign out of Windows 10 Technical Preview Build 10061 and return to the lock screen, you might see only a black screen with a responsive mouse cursor. Pressing the Alt+Tab keys or the Ctrl+Alt+Del keys has no visible effect.
If you are experiencing this issue, follow these steps to sign into Windows:
- At the black screen, press the Ctrl key once on an attached physical keyboard. (You won’t see any visible change, but pressing the Ctrl key will place the cursor in the password box that you’re currently not able to see.)
- Now type the password of the user that just signed out and press Enter.This will sign you back into Windows and the desktop will appear.
- If you failed to sign in because you mistyped the password, press Enter once more. Then retype the password and press Enter.
An update kb3055415 is available to resolve this problem. Download it from Windows Update.
More details about it here.