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 5, 6, and 7
In Parallels Desktop 8 and 9
In Parallels Desktop 10, the process name equals virtual machine's name:
Highlight this process, click Quit Process, and choose the Force Quit Option.
- Restart the virtual machine.