I assume this cause due to the Local Computer Policy settings of your PC.
To resolve it do the following steps it might work
Go to Start Menu Type gpedit.msc
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.
Might need to reboot
Hope that helps