Press CTRL ALT Delete on your keyboard. Go to Processes look for Minecraft.exe if there is more than one then close them all. I do not know if this works for Mac but it usually does for Windows.
Sometimes it says badlogin for me,but what batt said close miencraft and open it again thats what i do
CTRL + SHIFT +ESC - If you want to do it your way, then you still need to select the task mananger at that menu