My 24" iMac wouldn't start up after working fine. I tried a different power cord and that didn't work either. Came to the conclusion that the power supply had failed.
I ordered everything from ifixit and followed their step-by-step guide to get inside the iMac. I removed the old power supply, replaced it and guess what? My iMac is now good as new! Everything went perfectly.
Just go slow and follow each step carefully. My mac was actually a little different looking inside than the pictures with the guide, but pretty close. Be careful loosening the power supply cables so you don't break anything. These were in tight.
Heavy-Duty Suction Cups (Pair)
TR10 Torx Security Screwdriver
TR6 Torx Security Screwdriver