Could just be a bad or unseated module.
Try reseating the RAM modules, as per > iMac: How to remove or install memory
Besure to firmly push on each module with both thumbs, to insure that they are fully seated in the slots.
Then if it still does not startup, test for a bad module by installing the modules one at a time in one of the top slots. (the top slots with the iMac is facing display down, as per the installation instructions)