My iMac 27" I7 fully loaded audio production tool suddenly died. It would not power up at all and I could hold the power button down and nothing happened. I tried different outlets and power cables just in case. After a few moments of panic started to look online and do some research. I found out that my iMac was a 2011 model that was known to have power issues and a trickling effect of multiple components failing one after the other. I was really worried at that point, and thought it would have to be replaced. I bought it used, and it did not have apple care. I am an experienced computer technician but have rarely ever had to repair any of my Macs, so this was a little troublesome. I found the Ifixit site and began to read up on the failure. I felt it had to be a power issue since nothing happens when you pressed the power button and I found the parts and instructions for replacing the power supply inside the iMac. I ordered the parts and tools and crossed my fingers.
The instructions were spot on and fabulous! I had the old power supply out in about 20 minutes and that included reading and rereading the instructions multiple times to be sure I was doing the correct steps. I compared the power supplies once I got the old one out and was a little concerned because they didn't match correctly other than the shape. Coming from a PC world, not having matching parts can cause imminent doom when you power up. But after putting it all back together again, plugging it in, and then crossing my fingers... I pressed the power button. That wonderful familiar sound of the drives spinning up and apple boot noise sounded like music to my ears. It is alive again and kicking.
My advice is.... if your iMac won't power up and you don't have any warranty available to have apple fix it, give this a shot. The videos are simple and straight forward, the instructions on pdf are the same, and the parts just work.
Heavy-Duty Suction Cups (Pair)
TR10 Torx Security Screwdriver
iMac Intel 27" EMC 2309, 2374, 2390 or 2429 Power Supply