Can someone help me with a Gateway E4300 power issue?

I recently removed the cpu heatsink fan in my gateway e4300 desktops, and cleared them of dust. Upon reinstalling the fans into each machine, I found that none of them boot. The POST doesn't even show up. When I remove the 12V rail, and leave the main power connected to the motherboard, the system fans turns on, as well as the hard disks, but it does not boot. When the 12V rail is attached, the bezel light on the front panel turns on as well as the internal power light on the motherboard, but no fans turn on and the system does not do anything. This is happening in all three of my Gateway E4300 desktops. I've also tried replacing the PSU, but with no success. Any suggestions?