You can restart Macs running OS X Yosemite using either the Apple menu or a combination of the Control key and Power button. If your Mac does not respond to these standard restart methods, perform a forced reboot instead. Save any open files you want to keep before rebooting your Mac.
Rebooting Your Mac
The safest way to reboot your Mac is via the Apple menu. Click the "Apple" logo in the top-left corner of your screen and select "Restart." If you have windows or apps open that you want to reopen when the computer reboots, place a check mark in the "Reopen Windows When Logging Back In" check box. Click the "Restart" button to reboot your computer. You can also reboot your Mac by holding down the "Control" button and pressing the "Power" button. Click "Restart" to confirm you want to reboot your Mac.
Forcing a Mac to Reboot
Perform a forced restart if your Mac is frozen or otherwise unresponsive to the regular methods of rebooting it. Be aware that you will lose any unsaved files and data when performing this procedure. To perform a forced restart, press and hold down the "Power" button until the Mac shuts off. Wait a few seconds and then press the "Power" button again to turn the computer back on. You can also force your Mac to restart by pressing "Command-Control-Power."
Locating the Power Button
On iMacs, the Power button is found on the bottom back corner of the screen. On newer MacBooks, the Power button is located in the top corner of the keyboard. On older MacBooks, the Power button is located above and to the right of the keyboard. The Power button is identified by an icon that resembles a circle with a line jutting into it.
Closing Applications Before Rebooting
To prevent the accidental loss of data, close any apps and save unsaved files before rebooting your Mac. If you reboot the computer with applications open, the Mac may not be able to recover unsaved files and other data when it turns back on. If an app won't close normally, use the Force Quit dialog to force it to quit. Press "Command-Option-Esc" or click the "Apple" menu and select "Force Quit" to load the Force Quit dialog. Select the app you want to close and then click "Force Quit."