I went to power up my iMac g5 (which worked fine two days ago) and after hearing the startup sound, there was no picture, and the power light stayed on. After powering it off and trying to power it back on, It did the same thing, but about 30 seconds later the fan kicked on at full blast and stayed that way until i powered it off.
iMac, Mac OS X (10.5.8)
It would help if you narrowed down which iMac G5 you have:
http://www.everymac.com/systems/apple/imac/faq_g5/
You may just need a PRAM battery replaced. But you might also have a more serious problem with the power supply or capacitors as that series of models was prone to have. You may be lucky and have an authorized Apple dealer nearby that still has stock of the old exchange program parts, but generally speaking, that model is so old, that's really hit or miss.
iMac G5 no picture
