The question mark is telling us the system can’t find a workable OS to boot from. This could be due to a dead drive or one the OS has been damaged.
Given your systems age it can also be due to a power disruption so the systems PRAM settings got lost.
Typically, the PRAM battery Power Mac G5 PRAM Battery Replacement would hold the settings if it was still good. In addition, your system uses a vampire tap to the AC for a trickle charge so even if your battery was dead you wouldn’t have noticed! Now with a dead battery and the system was powered off Bang! the settings got lost. So… It may just need a new battery!
But how do we get it going now? Luckily we have an out! Here we will leverage the Startup Manager. Restart your system and this time press the Option key. Your system should now list the drives it has with a copy of OS-X/macOS on it. If not, then your drive is gone, if you do see it then select it using the keyboard arrows and enter.
Here’s a rundown on the Startup keys Mac startup key combinations
Here’s a run down of the different screens About the screens on your Mac as it starts up