Hi there,
Before we start troubleshooting I love to do basic resets first.
Some of them may seem unnecessary in this stage but I like to ensure that some weird glitch doesn't affect any future troubleshooting.
1. Unplug your charger and remove battery. Hold power button for 30sec.
(people call this cold reset, apparently this discharges any capacitors or so holding any current)
2. Battery back, charger plugged. PMU reset (power management reset). Press the power button and hold it for few sec, sleep light will rapidly blink and 50/50 chance that it will make non-Mac like beep.
3. Power it off and then back on - before anything came on the screen press Command+Option+O+F
This will enter into the OpenFirmware management screen.
Type reset-nvram Enter key, and reset-all Enter key. After this it will restart.
4. And now lets do PRAM reset. Command+Option+P+R - press and hold after powering on your Mac.
I do like to squeeze out 3-4 startup chimes. Some people say its too much but better a bit more than little too less.
Now we can start troubleshoot on why its not booting. (Still give a go to boot after all those resets, you never know)
Easiest way to understand why it's not booting and where it hangs is to boot your Mac into the Verbose boot mode.
Command+V on startup. Screen will turn black and you will see some Unix like booting.
Please let me know how far does it gets and what it says.
Make a picture if you wish. That's even better.
Thanks.
Regards,
Kris M