Most likely failure is the GPU. When it fails you may see it boot and the progress bar go to about 65-70% then it may shut down or shut down and reboot. I have one on the bench right now with the same issue.
This is a known issue and there was a repair program on it for a while.
https://9to5mac.com/2013/08/16/apple-ope...
On mine I intend to do a reflow of the GPU with a hot air workstation. I have had about a 70% success rate doing that.
The replacement GPU are still running in the $200-$300 range because of high demand. @ danj knows of some replacements you can try.
Personally I am looking at the Dell Alienware M17x R5 Ranger nVidia GTX 765M 2GB Laptop Video Card 9R3F5
There are some videos I am watching to learn how to make it Mac usable but I have not finished yet.
I emailed this distributor yesterday to ask if this one was already flashed since he stated it is for the iMac and costs about $30 more that the others:
https://www.ebay.com/itm/New-Nvidia-GeFo...
I will keep you updated on my progress if you want.
iFixit does not have a full guide on replacing this but this guide will get you close, I may take some pictures to do a guide while I am at it:
Installazione del Kit Doppio Disco nell'iMac Intel EMC 2429 da 27"
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What happens when you connect an external monitor does the external react the same way?
Have you tried running the onboard diagnostics? Restart the system and press the D Key are you able to see the display then? And what errors (if you got any) did you get.
da Dan