Troubleshooting: My computer won't turn on
Some things to try.
* Reset the PMU http://support.apple.com/kb/ht1939
* Resetting the Logic Board - Resetting the logic board can resolve many system problems. Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules.
1 Unplug the computer.
2 Press the Power On button on the front of the unit.
3 Open the side access panel.
4 Remove the battery from the logic board.
5 Wait at least 10 minutes before replacing the battery.
6 Make sure the battery is installed in the correct +/-
7 Reassemble the computer and test the unit.
This procedure resets the computer’s PRAM. Be sure to
check the computer’s time/date and other system parameter settings
* Resolve startup issues and perform disk maintenance with Disk Utility and fsck
* Your Mac won't start up in Mac OS X (Mac OS X 10.3.9 or earlier)
* Troubleshooting: My computer won't turn on
* Power Mac G4: Computer Does Not Power On, No Fan Movement
Your internal PRAM battery may be weak or discharged. Suggest you replace the battery. Check this site for battery part numbers and sources. https://sites.google.com/site/macpram/mac-pram-nvram-cuda-pmu-battery-tutorial Mac PRAM, NVRAM, CUDA/PMU & Battery Tutorial