Option-Command-Escape is a good combination to try. (Press them in that order, but don't let go of any of them until the last one is pressed.) This will allow you to force-quit any applications that have crashed. Any unsaved modifications will be lost if an application is forced to close. Your best bet is to wait a minute or so and see if you can typically close any programmes. Save what you can and restart your computer if you can't.

Before pressing Option-Command-Escape or the corresponding command from the Apple menu, make sure you're in the right place. (in the menu bar's upper left corner) If everything isn't working and you're seeing a 'spinning beach ball,' make sure you're in the 'Finder' by clicking anywhere on the desktop or pressing the Apple key (= command) and Tab until you see the Finder's icon (and all the rest of the open applications). Select it with the arrow keys or by holding down the Apple and pressing Tab again (or moving the cursor on it and releasing it).

If the force quit command fails, there are additional options. Activity Monitor is an application that can be found by searching for it in spotlight. It will provide a list of all open applications as well as information on the amount of processor power, memory, and virtual memory that are being used. You can quit a programme from within Activity Monitor if it looks to be taking too much CPU or memory, or if it is marked in red. Select the programme and click the quit button in the toolbar. This will allow you to choose between a conventional quit and a forced quit. If the regular quit method does not work, utilise the force quit method.

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