When you crash and must restart, sometimes you cannot get to the Desktop to choose "Restart" from the Special menu. (The "Restart" button on the alert box that tells you you've crashed almost never works.) Before you resort to actually turning off the power, try these:
On most machines you can restart by holding down the Control and Command keys, then pushing the Power key (the key that turns on your Mac if you have the kind of Mac that you turn on by tapping the triangle-marked key on the keyboard).
If that doesn't work, press the Reset Button on your Mac, if you have one. It's a little button somewhere on the front, side, or back of most Macintoshes. There are usually two buttons or tabs--the Reset one has a tiny triangle on it.
If that doesn't work, turn off the computer by pressing the little button on the back (oh, sometimes it's on the front) of the computer box (not the monitor). On some machines it isn't even a button, but a little plastic tab that presses in. It might look like this: (insert power 1) or: (insert power 2)
Some computers don't have a separate little button for resetting the machine--you have to use the Power button. For instance, on some G4s, to turn off the computer when it crashes you must hold the Power button in for five seconds, then push it again to turn the machine back on. On some G3s, the little light that glows on the front of the box to tell you the machine is on is actually a Power button--push it to turn the machine off when all else fails.
After you turn it off, wait at least ten seconds before you turn it back on.