Use the "kill" command: Open a terminal and type "ps -aux | grep <program name>" to find the process ID (PID) of the halted program. Then run "kill <PID>" to force the process to end. If that does not work, try "kill -9 <PID>" to apply the KILL signal, which should end the process unconditionally.
Use the "xkill" command: Open a terminal and type the command "xkill". Your mouse pointer will turn into a Turtle symbol. Just click on the hung program window to close it.
Use the system monitor: Most Linux distributions have a graphical tool called "System Monitor" or something similar. You can find it in the applications menu or run it from the terminal with the command "gnome-system-monitor" or "ksysguard" (for KDE). In the system monitor you can find the hung program and close it.
Restart the graphical environment: If none of the above methods work, you can try restarting the graphical environment. To do this, you can use the Ctrl + Alt + Backspace shortcut or run the command "sudo systemctl restart display-manager" in the terminal (where "display-manager" can be gdm, lightdm, or another display manager used by your system).