Unplug everything, take the cpu out, everything. Leave only the motherboard with the case attachments fitted - i.e. power supply connector, internal speaker, power switch, power LED, etc...
Then try switching it on. Will it beep? If it's an AMIBIOS, it should give one long continuous beep, which in this case would be interpreted as 'No CPU'. If you don't get a beep after a little while, check all the connections you've got in, especially the power cables and internal speaker.
If all that fails, you've probably killed your motherboard. If however you do get the beep, then Free Terran is right, you probably need a higher-rated power supply. This is relatively inexpensive.