If your Windows 7 or Windows Server 2008 R2 computer stops responding at a black screen if a screen saver is enabled, applying this hotfix may resolve the issue.
Lets say you have enabled the On resume, display logon screen option for the screen saver.
Now even when you use the mouse or keyboard, you are unable to turn on the display – only a restart helps you.
This problem occurs because of a race condition between the CSRSS thread and the WinLogon thread.
In such case apply hotfix from KB976427 to resolve this issue.