I usually do ctrl-alt-f1, sysrq-6 (to see the messages), sysrq-s (wait for the all sync message to pop up), sysrq-u (so no processes try to write to the disks again after the last sync), another sysrq-s (should pop the message quickly this time), sysrq-o, wait one minute or so, turn the machine on again (the last part is kind of superstitious but I feel all warm and fuzzy inside knowing that if the power fails, I have seen the machine boot from zero last time, so it "ought" to work).