Everyone knows you can reboot a Windows computer by using Remote Desktop, then rebooting from there (Usually some form of Start | Windows Security or simply via the Task Manager).
However, I had a problem where I could not remote into a machine. It was a Windows 7 headless workstation, used only as an RDP server. I remembered something about being able to manage a Windows computer from another one, so I did a quick web search and found http://www.wikihow.com/Remotely-Restart-a-Windows-Machine-Through-Command-Line,
The command is simple:
A window pops up, allowing you to type in one or more computer names visible to the machine you are logged into, and saying whether you want to shut them down or reboot them.
Worked like a champ. I logged into a Windows Server 2008 at the client site, went through the procedure, rebooted the other machine, waited a few minutes, then remoted into it (and did the maintenance I was supposed to do).