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I have recently upgraded to ubuntu 12.04. I have been having some trouble with lock screen lately. While the system is locked, I sometimes get blank screen without the lock screen. So I am unable to log back in. I have to shut down the system & restart once again. While this does not happen every-time, it nevertheless keeps happening from time to time. This usually happens when I leave the system running leaving the system to lock on its own. Can anyone suggest some measure. My keyboard shortcuts or manual lock screen are both working perfectly fine.

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Welcome to AskUbuntu! It sounds like this is a bug, which is typically offtopic for this site. Rather, bugs should be filed on Launchpad so the developers know about them and can work on a fix. I am also experiencing a similar issue, so I have already filed a report here: Bug #996770. Please mark yourself as affected there and provide any additional information there in a comment which the devs might find useful. Thanks! – Christopher Kyle Horton May 8 '12 at 21:20
This bug with missing lock-the-screen feature is not only affecting unity. gnome and kde are affected too. export DISPLAY ... as described above is not helping ... ? – user92450 Sep 24 '12 at 14:14
I agree with dschinn1001. It's a serious bug and I have no idea how to fix it. It affects Unity, Gnome, KDE, etc. It's a general Ubuntu 12 bug. – kedmond Dec 21 '12 at 18:18

If you can do Ctrl+Alt+F2 and login, then this might work:

export DISPLAY=:0; gnome-screensaver-command -d
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Disable VSync.

Anyway, you can type the password and press Enter even in the blank screen.

To solve this problem (white blank screen or black blank screen) on screen locker, disable the VSync option on Unity/Compiz or under Desktop Effects on KDE.

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