Standard debugging is to remove parts until the MB posts. You know that when it beeps.
Remove all USB + network attached device. No beep? Continue
Remove all PCI & PCIe cards from the PC. Do you get the no video beep? No Continue. (If it posted, then isolate which card made it fail. Sometime reseating the card fixes the problem)
Remove all memory from the system. Do you get the no memory beep ? Yes, isolate the failing DIMM. No, at this point all you have is the MB, case, PSU, CPU and wires from the MB to the power switch, front panel audio, etc. IF it doesn't post then you need to remove the MB from the case and try it with just the PSU, MB and CPU. If it doesn't beep then replace the power supply. If it still doesn't beep replace the MB. It's never the CPU.
Here is a great reference, you might want to follow it first. Be funny if you were just missing the 4 pin MB power connector. http://www.tomshardware.com/forum/261145-31-perform-steps-posting-post-boot-video-problems