Seconded. I use a PowerMac 8500 all the time and its PRAM battery is dead, so I have to set the date each time I power it up (I turn off the outlet strip to it at night). Recently it would not power up. No response to keyboard power key or button on front. Unplugged, plugged back in, no response. Pulled the logic board and tried running without any hard drives, same thing. Swapped power supplies, same thing. Tried a second logic board and that booted, then I connected the original and it booted as well. Conclusion: NVRAM (the stuff Power Manager uses, not PRAM) was corrupt in some way it didn't like, but it somehow got into a different state. I imagine I could have gotten it to boot by holding the "cuda" button inside, which resets NVRAM.