Do you hear boot-chime?
If yes, try to go to OpenFirmware console (during booting, hold down Command + Option + O + F until you've arrived at the Open Firmware prompt.). If you can go to OF, please post it - then we can check further.

If you dont here chime, you may have a serious HW-problem. That means that the low routines for checking basic HW-components fail. This can be mainly caused by CPU, PSU, RAM, Graphic-card or one oft the main components on motherboard. You can try to remove battery and press reset-button underneath RAM-sockets. Sometimes this may help....
In some cases PSU is the problem - maybe capacitors are going to be defect.