Currently Being ModeratedOct 27, 2012 12:19 PM (in response to lapresle)
In addition to replacing the PRAM battery as was suggested, you may need to use a wired keyboard (USB) to be able to use some of the special startup options (such as safe mode with shift key held down, which is a good thing to try).
Also, try resetting the SMC, which can be done as follows: shut down the computer, remove ALL connections to the computer, including the power cord, all usb and firewire peripherals, etc. Wait 60 seconds. Then, while holding in the power on button, plug in the power cord to the back of the computer. The release the power button and push it one more time to start up.
Currently Being ModeratedOct 27, 2012 1:39 PM (in response to lapresle)
If you power up the machine and press Apple key + V you should get text lines instead of that Apple logo, last line usually tells why/where the booting hangs. iMac G5 machines had lots of troubles with graphics card, I know for a fact that sometimes when graphics card is failing it might work just fine under very light load (eq. booting screen) but will hang when its acceleration features are enabled. Hopefully it is not hardware problem. One option would also be to try to boot from disc and use Disk Utility to repair the disk, you might also click repair permissions just to be sure.