There is https://en.wikipedia.org/wiki/Magic_SysRq_key but I've had some hit and miss success in actually getting it to work.

Normally I first try see if it is only the graphical interface that has crashed by pushing ctrl+alt+F1 or F2 or F3 to try and switch to another terminal. If I can switch, I log in and reboot or restart the window manager.

I also try SSH into the machine if I have another of around.

If I do reboot, then I use 'journalctl -b-1 -e' to see what happened at the time the system froze.