If no dialog box appeared in the first place, and the Macintosh is still frozen, or if the "Force Quit" button simply turned black and nothing happened beyond that, then you should try to force quit once again, and, that failing, reboot the Macintosh.
On the other hand, if you were successful, you should immediately save all your recoverable work, and restart the computer normally, as from the Finder.
If the Taskman extension is installed, hold the Control and Escape keys so that a dialog box comes up, select the application that has crashed, and use the button provided to quit it.