Mac OS "sad face" of old
Occasionally, a Mac can become stubbornly uncooperative and refuse to accept inputs. It's time for a reboot. Here are some simple techniques to force a restart your Mac with escalating abruptness.
The first thing to know about Mavericks is that it changes the behavior of the Power button. In Mavericks, if you quickly tap the Power button when the Mac is powered up, it will put the Mac to sleep.
However, if you hold down the Power button for 1.5 seconds, then let go, you'll get the Restart/Sleep/Shutdown dialog box.
An alternative way to bring up this dialog box immediately, if that 1.5 second hold isn't practical, is:
Control + Power button
... where the "+" sign means hold these keys down at the same time. If you elect to restart or shutdown, and if things aren't too hosed up, you'll be offered chance to save your work in each open application that's still responding.
However, if that doesn't work for some reason, and/or the dialog box fails to appear, and you need to force a restart of the Mac immediately, you can try:
Command + Control + Media Eject
There will be no dialog box as the Mac restarts, and you will generally lose unsaved work at this point. (This works even if you have a recent Mac without an optical drive.)
Finally, as a last resort, or if the Mac isn't responding to the keyboard, you can hold the Power button down for a full 5 seconds. That will force the Mac to completely shut down. Again, unsaved work will generally be lost. Then you can use the Power button to restart the Mac.
For those physically separated from the Mac, there are several ways to reboot a misbehaving Mac remotely. One well-known way is to try to SSH to the frozen Mac from another computer. It may well be that the mis-behaving Mac can still respond to the low-level SSH command. (System Preferences > Sharing > Remote Login must have been previously enabled.) Then you can enter:
> sudo shutdown -r now
You'll need an administrator's password to do this.
There are other methods as well, but this is getting a bit far afield for a single Quick Tip. Finally, for additional reference, see Michael Johnston's "Mavericks: Accessing the Shut Down Dialog via the Power Button."