Windows is frozen and unusable, I can't do anything inside the OS, however Parallels Desktop and Mac OS X work properly.
An issue with the guest OS.
This also happens to a physical PC. This might be caused by malware or Windows corruption.
To force restart a frozen Windows virtual machine:
Go to Actions > click Reset.
Note: In case this does not help or the menu of the virtual machine is not active, please follow the steps from the KB 6338