G5 iMac HELP!

Hi, I have a g5 iMac ambient light sensor desktop. Power button has always been tricky to get to come on. Unit will not power up at all! I removed back cover and know by led's power supply is good, unit will not come on by internal power switch. Any help on how to get back up again would be great! Has been a good computer, always ran fine til shutting down, which I did properly. Need a direction on where to go next. Thanks in advance!!