Your best hope to isolate the problem is to remove the HD. If you remove it and have some method, like a USB to SATA cable laying around and you connect it to another machine and it runs perfectly, like able to boot from it and everything, then you know you've got a faulty SATA cable. If not, then you know your drive is the issue and that's what you'll need to start salvaging information from.
Here's the guide on how to remove the drive - it's fairly complicated, and you don't want to damage that display. Depending upon your repair skills, you may want to contact a professional.
iMac Intel 24" EMC 2267 Hard Drive Replacement
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Please make sure your system info is correct here as well as fill in the details in your Q. The more info the better.
As you posted your Q in an old Q its' best to start afresh with your symptoms Vs hiding it in an old Q.
crwdns2934271:0crwdnd2934271:0 Dan crwdne2934271:0