Don't worry this happens all the time! Remember to power off before disconnecting anything. Do you get any beeps? 1 beep is good - more beeps is bad and can tell you what is going wrong.
First take out the motherboard and make sure that there not too many stand-offs; any extras can short out the board.
If this is OK, check you have all the front panel connections on the right pins - an LED connection on the reset button for example will stop it booting.
If this still does not sort it, disconnect everything and take out all the cards except the front panel and power supply cables. Does it boot? Reconnect one thing at a time and that will tell you what is causing the problem.
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