You may need to replace your internal PRAM battery.
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