Imac G5

Safe mode? Try starting while holding the Shift key.

iMac G5: Fails to startup

http://support.apple.com/kb/HT2173?viewlocale=en_US

Also, try this:

Resetting the SMU on a G5 iMac:

http://support.apple.com/kb/HT1767

and on resetting the PMU on a Power Mac G5, G4 and G3 :

http://support.apple.com/kb/HT1939?viewlocale=en_US

http://support.apple.com/kb/HT1436

It seems like the hard drive died or you have corrupted system files (maybe you force shutdown using power button and it corrupted the system). First of all, try safe mode (press Shift key while booting), and try resetting SMU and PMU.

If it doesn't work:

If you have a FireWire disk and system installation disk (or retail DVDs of Tiger or Leopard).

- Conect an external FireWire disk

- Boot up from your Mac's installation disk and install the system in the FireWire disk

- Boot up from the fresh installation in your FireWire disk (hold Alt key while booting to select boot disk)

- Then you should be in Mac OS X again. Copy all your files into an USB or something.

- Done! You can reboot again from installation disk and reinstall Mac OS X into iMac's internal hard drive.

If you haven't got any FireWire disk or system install disk, but you have another Mac with firewire, follow the next steps:

- Connect a FireWire cable between both Macs

- Turn on your iMac and press down "T" key while booting to access to Target Disk mode

- Now, the iMac's hard drive should appear in the other Mac. Copy all your files to your Mac or an USB.