Here are a few different options to try.
If the computer freezes during startup or you see a flashing question mark, wait a few seconds. If the computer doesn’t startup after a delay, shutdown your computer by pressing and holding the power button for about 5 seconds, until the computer shuts down. Then hold down the Option key and press the power button again to start up your computer. When your computer starts up, click the hard disk icon, then click the right arrow. After the computer starts up, open System Preferences and click Startup Disk. Select a local Mac OS X System folder.
You can try resetting the SMU.
First shutdown the computer. Then detach all cables, including the power cable, from the back of the computer. Wait 10 seconds, then while on the power on button, plug in the power cord. Release the power on button, the press the power on button again.
If that didn't work, try resetting the PRAM.
Shut down the computer. Turn on the computer, after you hear the chime, but gefore the gray screen appears, press and hold the following keys on the keyboard simultaneously:
Command (⌘), Option, P, and R. Continue tohold down these keys untilyou hear the start up chime two more times. Release the keys.
After resetting the PRAM, you may need to reconfigure your speaker volume, screen resolution, startup disk selection, time zone information. If this solves your problem, (but your problem later reoccurs) you may need to replace your PRAM battery. If you have an iMac G5 iSight, you'll need to take your iMac to an Apple Genius Bar, or an Apple Authorized Service center. If your iMac G5 doesn't not have iSight, you can replace the battery quite simply yourself.
Try this link for the PRAM battery:
You can probably find this battery at you local Walmart, Target, Walgreen's, CVS, Radio Shack, etc. But check your prices, there can be a wide range: macsales.com for $1.99 to Radio Shack $12.99.
If those options didn't work, try restarting off your original Start-Up CD/DVD that came with the iMac.
The following is an excellent link from a discussion on here that explains this procedure:
I hope one of these options help.
Very good infiormation. Thanks.
I am having the same problem with my G-5 (last edition) iSight iMac. (Running 10.4.11)
I took it into a Apple Authorized Dealer as the genius bar guys said, they no longer carry parts for the PPC's.
I created a disk image of my previous drive with Carbon Copy Cloner- so I have all my data, apps, etc.
They replaced the hard drive. I asked them to start it up before I took it home, and the fan kicked it and would not re-start. So then they replaced the ram.
I decided to do clean install of only the apps I need to run in Tiger (this is one of three machines I use in my studio, and have kept it to run Freehand and an older version of Photoshop that has several proprietory plug-ins for textile design.)
It worked fine during testing and all day yesterday.
This morning, I got the grey screen/Apple logo frozen screen with the loud fan.
1) I re-started with the original system disk, it does come up. Then I re-stared and it seemed fine.
2) I cold started with the option key down & selected the drive. And it seemed fine.
3) I also re-set the PRAM, it is seems fine.
But I don't think my problem is over, so unless anyone has any other suggestions, I think I need to take this back into the shop & try replacing the PRAM battery. Obviously the Apple Auth repair place, is not as smart as the folks in this forum (otherwise he would have tried that!)...
Any other thoughts?
My G5 won't even turn on now. Do you think it's done for? If I unplug it and then plug it back in it might turn on after many button pushes, but still will only get to the logo screen. I can't afford an expensive fix. that's why I'm trying to see if I can get it back to working myself. I hate to give up on it. It was a great machine.