It is impossible to stop/restart a virtual machine from the Parallels Desktop User Interface because all the buttons are grayed out.
On the Host computer, open the Activity Monitor application from Finder -> Applications -> Utilities and find the following process:
In Parallels Desktop 5, 6, and 7
In Parallels Desktop 8 and 9
Highlight this process, click Quit Process, and choose the Force Quit Option.
Restart the virtual machine from the Desktop Shortcut or Virtual Machines List.
If you're using Parallels Server, restart the Parallels Management Console.