Why wont my iMac turn on?

I press the power button and nothing....the iMac just wont turn on, it does receives power because I connect an external hard drive and I can see the external drive comes on.....now Im not much of an electrician, but Im not sure if that means the power supply is working....if the external drive comes on, but the iMac doesnt even makes a sound, it just stays off....I have however tried it on different wall outlets and samething, it just wont come on...I tried connecting it to a Mac mini and started on firewire mode, but it didnt come on, I believe its the power supply, but whats with the external drive coming on when I connect it to the iMac...i dont hear no fan, no nothing, any suggestions or advice would be greatly appreciated, thanks!!!