I am pissed that my iMac Mid-2011 27" (3.4 GHz i7, 16 GB RAM) just had a graphic card failure, but because the model just a few weeks ago got classified as "vintage", authorized Apple service partners are not allowed to service it, and there are few or virtually no spare parts.
Hey Winterflags,
I had a similar problem about two years ago on my 2009 iMac. So you can still start the computer in safe mode (i.e. holding shift)? If so, there was a somewhat common graphics card issue for 2009-2011 iMacs (before the thin-backed ones) where eventually the solder inside the graphics card comes undone. You can actually fix it yourself in a couple hours of your time and $10 in heatsink gel and heat transfer pads. (+$15 in screwdrivers if you don't already have them.)
I had the same issue since 2 years ago when my computer broke, it was also just vintage and no one would service it. And I had a quote for $650 even if they were willing to service it at a third party place here, whereas I did it myself for $10.
This of course depends on your personality and tinkering abilities.