Symptoms
It is impossible to stop/restart a virtual machine from the GUI because all the buttons are grayed out.Resolution
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
prl_vm_appIn Parallels Desktop 8
Parallels VM

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.
No
Yes