Computer will not power up

I have had this computer for 2 years. I replaced the 120 mm fan in the back of the case. The original fan’s power cord looks spliced into another cable, so I left it alone. I plugged the new fan into a 12V supply connection listed in the motherboard owner’s manual. When I went to turn the computer on, there was no response at all.

I have changed outlets, checked for loose cables, and verified that the motherboard is getting power via the light.

The only idea that I have is that the power button itsef may have come loose. I cannot access that part of the case because the case is riveted in many areas.

Just to be reiterate, when I press the power button, there is no response whatsoever. No fans, lights, nothing.