Either two things did this, either you are infected with a spyware that changed this value in the registry, or you were playing around with gpedit.msc and changed the policy to remove it from the start menu.
This is how you get it back
Click Start, Run.
Type gpedit.msc and click OK.
Navigate to the following path:
User Configuration\Administrative Templates\Start Menu and Taskbar
Double-click "Remove and Prevent Access to the Shut Down command"
Select "Not configured" or "Disabled"
Close Group Policy window.