This solution should be able to restore your shutdown button!
This cause due to the Local Computer Policy settings of your PC.
To resolve it do the following steps:
Go on Start Menu & Click on Run.
In the Run Dialog Box Type gpedit.msc and OK.
Go for the following option:
User Configuration\Administrative Templates\Start Menu and Taskbar
Double click on Remove and Prevent Access to the Shut Down command
Select any one of the option "Not configured" or "Disabled"
Shut Down the Group Policy window.
I hope this helps!
Windows Outreach Team