OK, it's been a while since I took a look at Unity, so my memory of it is a little hazy. But at a guess, you might want to check that the keyboard shortcut to lock the screen hasn't been changed or disabled somehow.
If you go to System settings->Keyboard->Shortcuts, then take a look for an entry for locking the screen. You might find that the shortcut has been disabled, or changed, or possibly removed.
If you get there and it says that ctrl+alt+L is still the combination and it's enabled, but it's still not working; try changing they keyboard shortcut to something else (like ctrl+shift+alt+L) to see if that works.
If that fails, then you've probably found a bug in Unity. In which case, you might want to report the problem to the Ubuntu devs.
The shortcut isn't disabled , this happened after I followed some steps written in an article titled "10 things to do after installing ubuntu 12.04 lts " .
sometimes it doesn't lock only the screen becomes white and nothing responds .
what shoul I do .