Shutting and Restarting Server Core

To shut down or restart Windows Server 2008 Server Core, you need to use the 
shutdown.exe file.

 To shut down the computer, use the following command:
shutdown /s

To restart the computer, use the following command:
shutdown /r /t 0 or shutdown /r

The /r specifies reboot, whereas the /t 0 (short for 0 seconds) indicates a
reboot immediately.

To logoff, use the following command:
shutdown /l