I am using ubuntu 10.04. Over the last couple of days, I am having a weird problem: once the machine gets into screen saver, it is impossible to get it "out".

I do see the mouse cursor move as I move the mouse, but the login popup (with user / password) does not show up and the screen remains "frozen". Any idea what might be going wrong?

For the time being, I have no choice but to restart the computer in order to be able to resume work. Thanks.