How to deal with a frozen screen?

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http://askubuntu.com – My OS is Ubuntu 12.04. Sometimes the screen suddenly becomes frozen, except that the mouse can move around. I have no choice but to hit the power button of my laptop. In Windows, whenever screen becomes frozen, I just hit Ctrl+Alt+Delete to invoke task manager, which is still responsive, and kill the processes that consumes most resources and are likely the culprit of the freezing. But in Ubuntu, I don't know what some good way to deal with such a situation. Thanks! (HowTos)