First off make sure power is arriving at the PSU, i.e. check or substitute the mains power lead. Check the voltage switch is set correctly for mains voltage 115v, 230v etc., as appropriate for your area.
Disconnect all drives, and this means both data and power cables. Remove all plug-in cards from the motherboard including video, unplug mouse and keyboard. Disconnect front panel l.e.d., reset and power switch wires from their motherboard headers.
Remove memory modules, and recheck CPU just in case there is an unnoticed bent pin etc. Put a fresh smear of heatsink compound on CPU and remount heatsink and fan. At this point you should now have nothing else connected to the motherboard except for the ATX PSU 20-pin power connector.
From your motherboard handbook identify the power-on 2-pin header, and either carefully short together these two pins or attach just the front panel power-on switch.
If powering on produces no spinning PSU or CPU fans, then either the board or the PSU is dead. Try removing the board from the computer case, to make sure there is nothing shorting to ground on the underside. It is all too easy to inadvertantly add a brass standoff where there is no motherboard mounting hole, which then can short out a vital circuit. Rest the motherboard on a piece of cardboard or similar insulating material and try powering it up out of the case.
To check the PSU, disconnect the ATX 20-pin power connector from the motherboard, attach a hard drive preferably an old one to give it some load, and short pin 14 (usually the green wire) to any black wire (there will be one either side of it). If this does not make the hard disk rotate or the PSU fan revolve, then the PSU is at fault.
ROGER - G0AOZ.