1.The first thing you can do is trying to reset the PMU. There is a small switch on the logic board board next to the battery that you can press. Wait 10 seconds then try starting the mac again.
2.If that doesn't work, then the battery may be bad. OWC (
http://www.macsales.com) sell replacement batteries for $5.
3.If the system still doesn't want to boot, then you might want to try re-seating any PCI/AGP cards you have.
4.Another option is to reseat the processor.
If all of that fails to boot the Mac then you may have a bad processor or logic board.