Have you done a PRAM reset, CMD+Option+p+r...
In fact, do 3 in a row, takes a bit of time.
If you hold Option or alt key at bootup do you get a screen showing boot choices?
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.
Here's the range of serial numbers of eMacs with bad capacitor problems to which TMM refers:
- G8412xxxxxx- G8520xxxxxx
- YM412xxxxxx - YM520xxxxxx
- VM440xxxxxx - VM516xxxxxx
How to find the serial number: