is this a branded(hp/compaq etc) PC or a generic system.
If you know what make & model it is, let me know and I will update you on error codes etc.
Make sure that you have only got the PSU plug / video card/ ram & cpu fitted, to m/board and that power leads to drives etc are also disconected.
if you have no display and no beep codes or either one, chances are you M/board has failed, but it may only be a minor fault which has halted the system completly.
good luck mate.
robin