[From the "Problems experienced by the MacFixIt editors and how they were fixed" department.]
Several weeks ago, before going on vacation, one of the MacFixIt editors completely unplugged all of his computers from AC power. When he returned a few weeks later, he plugged everything back in and booted up. A PowerMac G5 started up -- the drives and fans were spinning -- but produced no video. He tried the monitor with another computer, and it worked fine. He tried other monitors on the G5 with no luck. He went through the usual troubleshooting steps: booting off a CD, safe boot, zapping PRAM, resetting NVRAM, etc. Still no luck -- the G5 would not display video. Even using a different video card didn't help.
If you're using a PowerBook or iBook, Apple recommends resetting the Power Manager in such situations; however, apart from a few Knoweldge Base references to older Power Mac G4 computers, Apple doesn't really mention resetting the Power Manager for desktop Macs.
On a whim, the editor turned off the G5, opened it up, and pressed the reset button on the logic board (which resets the Power Manager). The very next boot, video returned and the G5 was back to normal, as if nothing had ever happened.
We would have left it at that, except that last week a MacFixIt reader emailed about a Power Mac G4 that -- ever since a power outage in the area -- would not start up properly: "when you push the start up button...it lights up but the computer doesn't start and when you release the button the light slowly dims and goes out." The reader had tried everything. We suggested resetting the Power Manager -- after doing so, the reader's G4 booted normally.
Although Apple only recommends resetting the Power Manager "as a last resort," keep it in mind if you ever experience similar startup issues that can't be easily remedied.