[naive]

Naive's easy sometimes solution. Ensure the cable is plugged in correctly, try swapping the monitor and the cpu's power cables. Try a new outlet. Are there any beeps? If so, count them. If not, what noises do you hear? Failing that, if you have multiple ram chips, try one at a time in each slot. If none of these things work try unplugging the optical drives and hdds and everything and see if it just turns on and you get monitor. If none of these things work, I would assume power supply or mobo.