question mark on start up

Thanks for starting a new question. As you have guessed, your machine cannot find an valid operating system. You did right in trying to repair it and then format it but it appears to be non-resurectable. Time for a new one. Your machine uses an IDE/ ATA or PATA drive not the current SATA drives. Here's a guide, while it may not be exact for your machine, will probably help you replace it with little problems. Remember to replace the thermal paste while you are in her: iMac G4 17インチモデル M9168LLA ハードディスクドライブの交換