As others said, double-check your connections.
Now, in all this did you ever disconnect the power cord at the back while making changes? Oddly, that might help. I have had this happen to me a few times: The computer is somehow shut down oddly, and when I push the front power button nothing happens at all. But if I then either turn off the main power switch on the BACK of the PSU OR disconnect the power cord and wait for about a minute or two before re-supplying power, then the front pushbutton works just fine! I think somehow the part of the BIOS that still has power when it's "Off" so that it can watch for the pushbutton gets locked in a loop and never sees the button push. A full power down and cold re-start seems to fix it.