You've probably done this but it's worth mentioning: reseat your ram and apply a good amount of pressure. Also ensure your ram is in the right slots. Check cables, especially 24 pin and cpu power cable. Your PC will not boot without these in properly. As well as check your front panel wires in the mobo.
Another thing you could try is holding down the power button. Sometimes on first boot this does the trick.
If none of this works, do you have another psu you could try swapping out to test on the odd chance its not working properly?