Woke up this morning to a dead iMac (early 2006) that had been left sleeping overnight. I had once read that a depleted logic board battery would prevent the iMac from booting. But I was skeptical that this was the problem, since the computer was asleep and hence had some power running through it. I was more worried that the power supply had blown. Still, I decided to open it up and take a look. This video was a very helpful guide. The most frustrating part was peeling back the membrane that surrounds the display - I ripped it in a few places, despite being careful. The four screws holding the panel were also a pain, since there's not much room to work. (You'll need a T8 screwdriver, BTW.) Long story shorter, changing the logic board battery worked. Total cost was about $12 for the CR2032 batteries at Radio Shack, which only come in packs of three, and a few bucks for a new T8 screwdriver thin enough for the job. Hope this helps somebody save some cash for the next iMac.