Apple's iMac has gone through three distinct variations: the colourful line of one-piece "original" iMacs, the flat-panel version and the G5 "all-in-one" desktop model. Although the first two models have mostly been retired and replaced by newer computers, there are a still a large number of the G5 models still in service. While having to manually reset the PMU (Power Management Unit) on the machines is rare, it is one step that should be taken when the computer is unable to complete the boot-up process.
Start by rebooting the iMac with no peripherals attached, as well as "zapping the PRAM," as taking these steps solves many start-up problems. To reset the PRAM, boot the computer while holding down the "Command," "Option," "P" and "R" keys, and allowing the computer to boot two or three times, hearing the start-up chime every time. Release the keys and see if the iMac boots up. If it doesn't, proceed to Step 2.
Unplug both the power and keyboard cables, and place the iMac on a table or workbench, screen down on a soft cloth to avoid scratching the screen. Check to see if the computer is still warm by placing your hand on the top of it. If it is warm (or hot), then allow it to cool off for 30 minutes or more before proceeding.
Remove the back of the computer by loosening the three screws on the bottom of the computer. Note that these screws don't come out all the way, even when they're fully loosened. Then pry the back off the computer by lifting on the stand.
Locate the PMU reset button, which is found on the motherboard on the inside of the computer. Use a nonmetallic object (a pencil, your finger, a chopstick) to press the button in and hold it for about 5 seconds.
Replace the back cover on the iMac, and make sure the tabs in the top of the cover are seated properly in the slots at the top before tightening the screws. Plug in all of the cables, and reboot by pressing the power button on the back of the iMac.
Confirm that there are no CDs or DVDs in the drive when starting up. You can eject any CDs or DVDs by booting the iMac and holding the mouse button down. While you're inside the computer, use a can of compressed air to clean the dust out.
Static shocks can fry computer components, so wear a static strap. There are a few razor-sharp surfaces inside the iMac, so watch your fingers. Don't poke around inside the iMac with metal (or any) objects.