iMac will not even start to start.

I need diagnostic help!

My 20" iMac G5 (no camera) worked great until one day it just would not start. Pressing the start button yeilds nothing except (sometimes) a very short, quiet squeak sound. The power light does not come on. The screen does not light up.

I would like to fix this but am afraid of spending money on a power supply only to discover it is not a power supply problem. Can anyone tell me the likely cause of this problem, and how likely that it is the actual cause? Any recommendations or guidance would be very much appreciated.