It is impossible to stop/restart a virtual machine from the Parallels Desktop User Interface because all the buttons are grayed out.
On your Mac, open the Activity Monitor application from Finder -> Applications -> Utilities and find the following process:
In Parallels Desktop 10, 11 and 12 the process name is the virtual machine's name:
Highlight this process, click X at the top left corner and choose the Force Quit option.
- Restart the virtual machine.