Nevertheless, if you need to take control and gracefully restart a completely "frozen" Linux box, you'll be happy to know that there is hope. Hold down the ALT+SysRq key combination. While holding down ALT+SysRq, press R-E-I-S-U-B and watch your system reboot gracefully (e.g., no corrupted file systems). Here's what R-E-I-S-U-B does while you're holding down ALT+SysRq ...
- R: Switch the keyboard from raw mode to XLATE mode.
- E: Send the SIGTERM signal to all processes except init.
- I: Send the SIGKILL signal to all processes except init.
- S: Sync all mounted file systems (IMPORTANT).
- U: Remount all mounted file systems in read-only mode.
- B: Immediately reboot the system, without un-mounting partitions or syncing.
Check out some of the other Linux ALT+SysRq helper functions here.