While connected to a computer over RDP, the Shutdown or Restart options are not available in the Start menu.
To shutdown from command-line:
$ shutdown /s /t 0
To restart from command-line:
$ shutdown /r /t 0
PS: Curious to know why Shutdown/Restart are not available over RDP? See "Logoff" and "Shutdown" Are Missing from the Start Menu When You Use Remote Desktop.