Are you sure that you plugged everything in correctly? Not everything goes in the same. For instance, if you're moving stuff from a prebuilt computer to a new custom case, you now have to account for where to plug in the LEDs, front buttons, etc. It's stuff you might not have had to worry about before.
If everything is plugged in correctly (which it's probably not at this point), there's no reason you shouldn't be able to boot up. I remember the first time I did this, I plugged every stray plug/cable into things that looked like it would fit, and obviously, that wasn't the smart way to do things.