REISUB is used most often, to safely reboot a stuck up system. But it is not the only possible combination.
Normally sysrq does not work when X is running. If a ctrl+alt+fX does not work, you can try sending 'R' before it. If unsuccessful, sometimes a "saK" with 'K' fixes things; It kills all processes within current runlevel, including the X server. Then you can try other sysrq commands. After a saK to the X server, you can even login from a tty if the system is still running.
And sometimes when a silly process freezes the boot, when ctrl+C, ctrl+Z or ctrl+alt+del do naught, an 'E' followed by a ctrl+alt+del can do a safe and normal reboot.
'T' command prints a trace so you can inspect which process or function is causing problems.
See alt of the commands by pressing alt+sysrq+H