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I'm using Ubuntu 11.10 and a very novice user.

From today, I'm suddenly experiencing this problem. If I press Ctrl+Alt+L to lock the PC, I find the following error message

Couldn't execute command: gnome-screensaver-command --lock Verify that this is a valid command

and PC doesn't get locked. If I try to lock from menu item "Lock Screen" in the power button at the top right corner of the screen, nothing happens.

Yesterday I updated and upgraded the OS. I think this is the first time I'm trying to lock the PC after update.

What's going on? How to fix this?

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up vote 1 down vote accepted

You can try reinstalling the gnome screensaver by executing:
sudo apt-get install gnome-screensaver --reinstall
and see if the command and lock is available again.

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