Bummer. I know it's frustrating.
Couple of questions:
1. You said you were building it, but it sounds like everything was working in a different case, and you merely put it all in a smaller case, correct? No new components added?
1. I'm sure you have done this, but is everything seated correctly? Power supply to ATX cable snug and fully inserted on each side (if modular), RAM seated? You know the drill.
2. Are there any beep codes? Does the mobo have a error indicator LED or a number display?