Start at the beginning.
Unplug everything from the motherboard except the PSU, the video card, the RAM and the connections to the front of the case for the power button and see if it starts, enough to get into BIOS anyway. (i.e. HDD, DVD, USB connections, any extra cards (sound card network card etc - if applicable)
If it still won't start unplug the PSU from the motherboard and then do the 'paper clip' test to see if the PSU starts up.
Here is a link that shows what to do.
If it starts OK, use a Voltmeter to test each of the output voltages from the PSU to ensure that they are OK.
If that is OK use an Ohmmeter to check the continuity of the power button wiring from the button to the motherboard. Disconnect the power button from the motherboard to do this. Do this with the power disconnected from the PSU
If it is OK and it still won't start, check that the CPU is seated correctly and hasn't bounced out.