if the green light on the back of the PUS is solid then theres power going to the PSU. you need to chekc all wires mainly the 25 pin ATX cord to MB. if all else fails PSU are cheap now a days you can replace it for less than $50.
you can try to make sure everything is off and take the plug out of the pus on the back and then plug in it. if it dont turn on you may have a short somewhere.