Problems booting. "Can't find OS" flashing icon appears.

Question

I aquired an iMac G5 17" non-ALS, non-iSight in non working condition. I was told it was a power supply issue, and it was. I have since fixed the PSU problem, and have semi-successfully booted once to the default user's desktop ONCE.
On that boot, I was unable to get any response from the software. When I shut it down, it didn't completely power off. The fans continued to run full blast until I pulled the plug.
Since then, I have not been able to get past a completely gray screen (not even an apple logo). if I leave it there to think, I come back to the icon that alternates between the finder icon, and the question mark.
I tried holding option for a bootable device, but it sits there with fans full blast, and no bootable devices appearing.
I tried putting in an install CD (of a previous version of OSX) and booting with 'C' held, but nothing came of it. Same alternating icon.
Anything could help,
Superd@ve

Answer 1

Superd@ve wrote:
... A G5 should be able to run 10.5 right?
I would think so. To be certain, check your iMac's specs against the Leopard requirements:
  http://support.apple.com/kb/SP517
also, the stated 'previous version' was 10.1, so i can't see why that won't work...
My comments regarding compatibility were, indeed, concerning your statement that
I tried putting in an install CD (of a previous version of OSX)...
Your 10.1 is quite possibly a problem. My first OS X iMac came with 10.1. It was so buggy and hardware specific that I could not use it with most of my other system elements. I returned the computer and did not buy another OS X Mac until they were being shipped with 10.3 (a good, stable OS X that I still use.)
I see that user K Shaffer has given you lots of detailed information and specific suggestions that should help you understand why 10.1 is probably not a solution for you. Here, too, however, the suggestions are based on using the software disks that came with the iMac.
Note that some who post in Discussions advise caution when using third-party Apple info sources such as http://www.chipmunk.nl/klantenservice/applemodel.html. For one such example, see user R C-R's post here:
  http://discussions.apple.com/thread.jspa?messageID=11499980&#11499980
If you have security concerns, you may wish to has an Apple-Authorized Service Provider (AASP) for help identifying your Apple hardware.
... I doubt the previous owner has any disks as he has several computers of his own...
Check with the original owner. Owners of multiple Macs almost surely know that the bundled software disks are machine-specific. Thus, these owners are probably as protective of their original disks as I am of mine. As I noted earlier, if necessary, you may be able to obtain replacements from Apple if you provide them with the serial number of the iMac.
I would try to re-install the OS, but it doesn't have a DVD drive, and I have no other apple devices that can mount an SATA drive internally or externally...
I have never seen a G5 iMac that did not have a DVD drive. If your iMac is one of the models shown here, it should have an optical drive capable of using the DVD.
  http://support.apple.com/kb/SP69
If your Mac does not have an optical drive, and if the alternatives suggested by K Shaffer don't help you, an AASP can get your Mac working for you.
Perhaps you mean that your Mac's optical drive is not working. It is possible that your iMac might require extensive repairs. Consider obtaining an estimate of the repair cost before authorizing repairs by an AASP.
If you think the problem is with your iMac G5's hardware and you want to do the work yourself, you might get better help by searching or posting in the iMac G5 Forum:
  http://discussions.apple.com/category.jspa?categoryID=134
EZ Jim
Mac Pro Quad Core (Early 2009) 2.93Ghz w/Mac OS X (10.6.4)  MacBook Pro (13 inch, Mid 2009) 2.26GHz (10.6.4)
LED Cinema Display  G4 PowerBook  1.67GHz (10.4.11)  iBookSE 366MHz (10.3.9)  External iSight