Had the same issue with a 2009 iMac. Many months later I finally got back to investigating this. I removed the LCD panel again, carefully disconnecting everything. With the LCD off and the RAM removed, I powered-on and everything fires up and you get the constant beeping (which I believe is the 'no-ram' notification). I installed the ram and tried again. It turned on once more! At this point, with a friend's assistance, I held the LCD in place and connected one cable at a time -- then powering up. The result was that the LCD powered back on before I put the V-sync cable in (the last cable you usually connect.). So the result for me was that the Vertical sync cable was damaged which caused the no power, no fans, no nothing, issue. That makes no sense to me but I put the iMac back together without the V-sync cable attached and everything seems to be running fine. Since I don't do any graphic-intense activities on this iMac, I think I'll just leave it disconnected. Hope this helps others in the same boat.

I’m having the same issue, here’s my scenario: Bought a used late 2009 iMac in mint condition. Reinstalled OS, etc., its been working great. Decided to upgrade RAM and hard drive to ssd. I have experience replacing drives, broken ipod/ipad screens, etc.. I followed the iFixit guide (also reading through all the comments) as well as watched the OWC video. While I had the iMac open I cleaned out dust with compressed air and vacuum. Before people freak out, the vacuum did not touch any components. I hold the nozzle of the vacuum near where I blow the compressed air in short bursts so it sucks up the airborne dust. I have used this approach many times with zero issues. Went to install new SSD and realized I had wrong adapter ( was using one I had on hand). Reinstalled original drive but now no power when I turn it on. No fans, no nothing. Tried resetting SMC but nothing. Thought maybe I had blown the power supply so took that out to inspect but looks fine (not that I would’ve been able to see anything, but was...