do you get any beeps from the speaker at all?
If not, disconnect all drives, remove the RAM and all cards such as video. Reboot - you should get a string of beeps. Also all the cables to the front panel. Power off.
Replace the RAM and reboot by shorting the front panel power cable connection. Should be more beeps. Power off
Replace the graphics card and reboot, should now be 1 beep. Power off and reconnect the drives one at a time. Reboot after each reconnection - keep replacing things until you find what stops it.
If nothing at the beginning, take the motherboard out of the case and try it on a non-conducting surface - maybe a short to chassis somewhere.