It is impossible to stop/restart a virtual machine from the Parallels Desktop User Interface because all the buttons are grayed out.
For such situations it's required to force quit the virtual machine process using Activity Monitor application:
On your Mac, open the Activity Monitor application from Finder -> Applications -> Utilities and find the following process:
In Parallels Desktop 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.