Kill the task on both.
In Windows, you do this through Task Manager (hit CTRL+SHIFT+ESC).
In OSX, you do it through the Force Quit menu (CMD+OPT+ESC).
Then you select the program that's not responding, and click "End task."
You could also do it through the native CLI (CMD or PowerShell in Windows, the Terminal in OSX).
tasklist /fi "STATUS eq NOT RESPONDING" in PowerShell
I don't know what command you'd run in OSX.
