Cannot get past flashing question mark on iMac G4 flat panel. How can I deal with this problem?
That means the Mac can’t find a bootable drive/partition. Either the system software installation is damaged (software issue) or the drive is faulty (hardware issue). If you have an external drive with a system on it, try booting from it. Otherwise, try booting into Recovery mode. Start the Mac and immediately hold down Command-R to bring up Recovery. You must be connected to the internet to be able to reinstall your system.
Another step to try is resetting your SMC. If I recall correctly, on your G4 iMac, with it turned off, press and hold the power key for about 15 seconds, then press again to start up. If that fails, shut down again, unplug the power cord, wait for about 15 seconds before plugging it back in and pressing the power button.