First unplug the AC charger and remove the battery then hold the start button down for 5 seconds. Plug it back in and try. Next hold down the command & option and P and R keys and hit the start button, wait till it tones three times.

Try starting it up from the installation DVD. Insert the disk, hit the start button and finish pushing the disk in. Immediately hold down the "C" key. If this fails tell us what happened incuding any sounds or lights and we'll go from there.