Don't panic- I've seen it more times than I care to remember where I hit the power button and a system doesn't post, but later it does after I toggle something that was loose.
Double check all your connections. For the power-supply connections, make sure both are pressed in far enough for the clips to connect. Even if they look connected to you, pull them out and connect them again to be absolutely sure.
Most of all- make absolutely certain your RAM is seated properly. When installing, the hammer-clips at the slot ends should be pulled fully outward and when the RAM is pushed completely into place the clips should also click into place. Often a system won't boot with the RAM improperly seated even slightly. Again, if it's not posting after double-checking the power connectors- reseat the RAM even if it looks perfectly seated to you.
(Also: after checking everything else, try installing just one stick of RAM and not the other and seeing if it will post with just one installed. It's rare, but a bad RAM stick can tank an entire system. Chances are that it wouldn't be both sticks.)
Double-check your front panel connectors. I'd plug in the reset switch just in case.
If still out of the case, turn your board over and make sure the plastic tabs holding the heatsink are fully pushed through. The black pins should be obviously expanding the white outer connectors. I've seen occasions when an improper heatsink install makes the system auto-shutdown before it overheats.
If all the obvious easy stuff still fails- are you 100% certain you installed the CPU properly in the slot? (Sometimes it can be confusing, but the two notches on the slot and CPU should line up.)
Hopefully it's something obvious and simple and you can figure it out and get it working.
