cas123

What color is the power button light went you turn it on? if its amber then the psu is bad.
If blue unplug dvd, hdd, any PCI/PCIe, even the video card it have one(just need to heard the fan slow down = boot ok), memory card reader(unplug the usb header on MB),  then try boot.
If still no boot then try re-seat the Ram, try with 1 stick, on different slot, or use one from a different computer that you know it work 100% and try booting each time.
If still no boot then remove heat-sink and cpu, check the socket for bent pin or stuff stuck in pin, look at capacitor on MB is any went bad.
If all look ok, carefully put back the cpu(match the triangle) and heat-sink, rest the bios(unplug main power, remove the little battery, use the jumper and wait 5min then put back the jumper to normal, battery and main power).  After try boot.

IF all fault then you need to test MB, CPU and PSU on other system.