Computer problems.

Hello,
I've put together many computers in the past, but I've never ran across a problem quite like this. I recently took all my hardware out (RAM cards, graphic card, wireless card, sound card) to do a routine cleaning. Upon putting everything back, and hooking all my wires back in, it seems something has gone wrong. Now I must mention, before I took everything apart, I DID shut down properly and flipped the switch on the PSU and unplugged it.
Once I plugged everything back in, and as soon as I plugged the PSU in, the whole computer turns on.
The PSU does turn on and so does all the fans in the tower, the dvd tray has power and opens, I can hit the illuminate button on the keyboard, but the mouse doesn't work, the monitor doesn't work, and upon hitting the numlock button on the keyboard, nothing happens.
When I have the PSU plugged in, upon hitting the power button, the PSU stays on (fan running), but everything else on the computer shuts down and won't turn "back on".

This has never happened before and I've checked every wire, every plug. All RAM cards work perfectly, my graphics card light luminates, wireless card shows activity.
I just don't know what the problem is.
And I've been told to steer clear of the Best Buy Geek Squad on an issue like this.

ANY help would be greatly appreciated.
Thanks.