Doesn't sound good. Those systems are from the bad cap era.
Lay the system on its face, and there will be three captive screws on the bottom. Loosen those, then grab onto the stand and give it a good upward pull. The whole back should pop off.
You then need to inspect all the capacitors for any that may be bulging or leaking, which I suspect you will find. Google bad caps if you need a visual reference.
If you find any bad capacitors, the system is toast. Sorry. There are "repair kits" on ebay for about $20, but unless you know someone who's pretty good with a soldering iron, it doesn't do you a lot of good, and getting this repaired would probably be at least $500. That's about half the cost of a brand new 21.5" iMac with significantly better specs.
I have an iMac G5 iSight. When the power button is pressed there is no sound and a white light flashes, but nothing else happens. I tried the power reset method and replaced the power supply, but the same thing happens. The white light just keeps flashing. Any thoughts on this?