SymptomsIt is impossible to stop/restart a virtual machine from the GUI because all the buttons are grayed out.
ResolutionOn 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
Highlight this process, click 'Quit Process' and choose 'Force Quit' Option.
Restart the virtual machine from Desktop Shortcut of Virtual Machines List.
In case of Parallels Server, restart the Parallels Management Console.