Do you get any beeps when you attempt to switch it on, and if you do, what do you hear?
After each step described below, power your system up and note if there is any sign of life. Please report back with your results.
1) Check and recheck ALL motherboard connections, especially the power and front panel cables. If your motherboard has onboard graphics capability, remove your card (if you have one fitted) and use that instead.
2) Unplug the optical and HDD drives from the board and disconnect their power cables.
3) Reconnect the drives, one at a time. Repeat until all drives are reconnected.
4) If you removed a graphics card, reconnect it. Don't forget the auxiliary power connector(s) if needed.