To perform a safe reboot of a Linux computer which has otherwise locked up, the mnemonic “Raising Elephants Is So Utterly Boring”, or simply remembering the word “BUSIER” backwards, is often useful.
It stands for Raw (take control of keyboard back from X), tErminate (send SIGTERM to all processes, allowing them to terminate gracefully), kIll (send SIGKILL to all processes, forcing them to terminate immediately), Sync (flush data to disk), Unmount (remount all filesystems read-only), reBoot.
These keystrokes should be entered a few seconds apart. This should prevent a fsck being required on reboot; it also gives some programs a chance to save emergency backups of unsaved work.
To execute it hold down ALT+PrtSC (SysRq) and then hit the other characters with a second or two between: ALT+PrtSc + R+E+I+S+U+B