Is Ubuntu freezing when you try to quit? I have a probable solution for you

When I re-installed Hardy recently I went through System > Preferences > Sessions and disable a number of things I don’t need running in an attempt to get this older, rather underpowered computer running a little better. Unfortunately I disabled one thing too many and my system started freezing on me when I tried to Quit (shutdown, restart, etc.). While looking for signs others were experiencing the same issue I came across the bug that has a fix for the issue.

Comment #10 by APREA Thomas on Bug #150846 Quit button freezes screen but does not present Log out etc. options dialog suggests going to System > Preferences > Sessions and making sure Power Manager is set to run automatically. I made that change and tried a reboot but my system locked up again. I forced a shutdown of the X system by hitting Ctrl-Alt-Backspace and logged back in, and as I was responding to a forum post about the issue I did a check and saw that gnome-power-manager was running so I tried to call up the Quit dialog and everything worked properly.

So the solution is:

• Use System > Preferences > Sessions to make sure the Power Manager is set to run automatically every time you start Ubuntu.
• Use the System Manager to ensure that gnome-power-manager is running, and if it isn’t use Alt-F2 to get a Run dialog and run gnome-power-manager from there. (You can also run it from a Terminal.) This step is only needed during the session that you perform the first step.
• Attempt to Quit using either the Quit tool on your panel or by using System > Quit. You should get the Logout/Restart/Shutdown dialog.