iMac sortof powering up

My iMac G3 was working but now isn’t. It’s exhibiting bizarre behavior so I’ll be specific. When I plug it in I can hear a faint high pitch whine from the inside. Pressing the power button results in it glowing amber, the hdd spinning up, then the bootup chime. The display does not come on. Shortly after the chime there will be a sort of thunk then the power button will turn green within a blink of an eye the whole thing shuts off. It won’t even think about showing signs of life afterwards. The only way to repeat this is to leave it unplugged for an amount of time. When I plug it without doing anything else the lights will slightly flicker so I think something is happening.