G5 iMac won't start up

I have a couple-year old iMac G5. It was bullet-proof until last week when my son shut it down, moved it and attempted to start it back up. Nothing. We could hear a little movement in the HD but no image, no power light, nothing.

I took it into our IT guy and he replaced the small battery inside and tried to diagnose the problem online per the LEDs inside the iMac. He got it to boot a couple of times, but we can't seem to get it to start consistently. We're wondering if it could be the power supply.

Any thoughts? Anybody?

What did the lights tell you? Sounds like the Logic board to be honest. Hope you have Apple Care mate :-(