
Open OnyX, and type your password.

Click the Parameters icon in the toolbar, and select the Login tab.

Deselect the "Turn off the Restart, Shut Down, and Log Out menu items in the Apple menu" box.
If the Restart, Shut Down, and Log Out menu items in the Apple menu don't work, the Kiosk mode is probably turned on:
Open OnyX, and type your password.
Click the Parameters icon in the toolbar, and select the Login tab.
Deselect the "Turn off the Restart, Shut Down, and Log Out menu items in the Apple menu" box.
This issue sometimes occurs after updating the system.