/proc/sys/kernel/sysrq if it exists and contains the value 1, all possible requests are allowed.
Press Alt-Sysreq, then one of the following letters:
|r||Unraw: Restores the keyboard after an X crash or similar.|
|0||Changes console loglevel to 0 and so reduces error messages.|
|k||System attention key: Kills all processes on the current virtual console.|
|e||Terminate: Kills all processes except init on the current terminal.|
|i||Kill: Kills all processes except init, everywhere.|
|s||Sync: Attempts to sync all mounted filesystems. Outputs OK and Done when it's managed. This can reduce the chances of needing to run fsck at a later stage so it can be useful if you're having disk problems.|
|u||Umount: Attempts to remount all mounted filesystems read-only.|
|b||Reboot: immediately reboots the system without syncing or unmounting disks. Not a good idea unless in extremis! This may lead to data loss.|
|p||Dumps current registers and flags to the console.|
|m||Dumps current memory info to the console.|
If this doesn't work, check the documentation at /usr/share/doc/linux-doc-2.6.18/Documentation/sysrq.txt.gz for some suggestions (including checking the keycode being sent).