It is hard for experienced users to understand the severity of your problem, until it happens to them. PMU resets, PRAM, etc., as old comm guy writes, can be done before starting up on an alternate disk -- like your install CD -- but if they don't work, go back to the no-brainer, restart on your install disk.
What you describe is your problem happened to me, and I was able to do an NV-RAM restart, booting cmmd-option-N-V, and it found a big problem with my internal start-up drive. It tried to fix it, logging some of the following:
"checking disk
Invalid node structure
(4. 18459
disk0s3:I/O error.
**/dev/rdisk0s3
** RootFile system
** Checking HFS Plus volume.
** Checking Ex
Rebuilding Catalog B-tree.
Volume precinct could not be repaired. Killing all processes."
And it powered off the computer the same way that happens without the NVRAM reboot. There is nothing I can find to get to first base with this drive, including Disk Utilities (when booting off an install CD). Not even Erasing the disk with Disk Utilities. I'm out of options. The disk is incorrigible to all attempts to access it. I have to replace the internal hard drive, having lost all the data.
Your problem was four years ago, but perhaps this will help others who have the problem, and they can find a way not to lose their drive.