try booting into open firmware. i was working on a friend's 6116 CD (yeah, they're going for 5 dollars and less on ebay, but this is their only computer), and nothing seemed to be working.
it would turn on, spin up the hard drive, but never got past that. no video, and it didn't load the OS.
after re-seating the pram battery, resetting the mobo, and re-connecting everything i could, it still didn't want to boot. I couldn't even pull off a pram reset, cause the keyboard seemed to stop responding after the first 1/2 second or so.
so as a last ditch effort, i attempted to boot into open firmware (command option o f). it didn't actually start open firmware, but that combination suddenly made the computer start loading the OS. i had video, and within 30 seconds, i had one happy mac.
So give that a shot, even though i couldn't figure out what made it work. and it's been booting just fine since then...
matt