Does the machine post ? do you hear any beeps. If so how many ? do any of the fans spin up ? Some things to try
1. Pull the plug so it has no power then hold the power button down for 60 seconds. then release it and plug it in and try to boot .
If this gives no joy
2. Remove all but one stick of ram and try to boot that way try with each stick individually .
Still no joy
3. Replace the power supply with a known good supply
Hope this helps