You can do this the quick and step-by-step way, or the quick and dirty way:
You can follow the steps http://www.tomshardware.com/forum/261145-31-perform-ste...
You may have a bad power switch. Pull the connector from the motherboard header and cross the two pins for a fraction of a second with a metal object. If it boots, replace the power switch.
You may have a bad power supply. Remove it, replace it with one similar. If it doesn't boot, you may have a bad motherboard or CPU - the latter is not likely.