You could study the following sites for a solution:
G5 iMac fails to start – http://support.apple.com/kb/HT2173?viewlocale=en_US – if you've iSight built-in, this may not be of help to you.
Reset the System Management Unit (SMU) – http://support.apple.com/kb/HT1767 or http://support.apple.com/kb/HT1767?viewlocale=en_US&locale=en_US and http://support.apple.com/kb/HT1436
Reset the PRAM – when hearing the first chime on powering on, press the Command-Option-P-R keys together till you hear the chimes at least three times more, then release the keys, press the Shift key to boot into safe mode.
Reset NVRAM, booting into firmware – on powering on, press Command Option-O-F keys together till the white background screen appears and enter the following three commands in sequence, after which the computer should restart:
Hope this helps you get over the problem.