you didnt mention so i guess ill bring it up.

did you push that little button on the motherboard thats right by the battery? give that thing one push, then close it back up and try adn start it up.

if that doesnt work, pull that battery out for like 30 seconds to a minute and put it back in and try and start it again.

a similar thing happened to me a few months ago and the steps above brought my powermac back to life. i then promptly went out and bought a proper belkin power strip with a good amount of surge protection.