Hard system wide lock ups are 'usually' attributed to hardware.... or privileged driver code executing in kernel space and correcting kernel state.
A few checks can tell you "how hung" it is. Check if the caps lock/numlock responds on your keyboard. If it doesn't then you whole kernel interrupt mechanism is locked up too. It's a "RESET" or "5 second salute".
In Linux I have had the GUI crash so badly it took the keyboard with it. However, I was able to login via SSH and re-init the GUI and recover. Windows doesn't provide many ways to "Drop back to the basic VGA console" like Linux does though. (CTRL+ALT+F1-F10). The key combo to restart the display driver is about all you have. Linux has another combo, sequence that I can't remember, something about Raising Skinny Elephants", which the kernel itself captures and, IIRC will drop you back to a basic kernel debug shell. Usually if it gets that bad, I just reboot. Examples of where this happens in "untuned and unhardened" linux installs is when you hit a load average of 100+ and the application event queue is longer than a TCP SYN timeout.
X11 'used' to have a "Interrupt/Term" signal bound to CTRL+ALT+Backspace which would send "SIGTERM" to the X11 server directly. Killing it instantly. "init" would then respawn it immedaitely. This was a good "GUI layer" reset. It's been deprecated now, same as CTRL+ALT+DEL being shutdown, don't ask, just do it. These are all now 'trappable' in user space via desktop interactions and (can) pop up "Are you sure?" and options screens.