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 looking for obvious signs of damage). Reinstalled power.
Following advice of other commenters above, I removed RAM and disconnected HDD. Turned power on and low and behold it started with fans spinning, etc. However I had not reinstalled LCD.
After shutting it back down I reconnected everything , reinstalled RAM and went to power on. Same issue, no power. Just silence.
Do you all have any words of wisdom? Is it the HDD SATA cable somehow? It looks fine but I did have a tough time removing the SATA data connection when I first took the original drive out. The cable itself seems more robust than so many of the other cables in there.
I would greatly appreciate your thoughts.