First, check the connections are okay, the power to any power strips are working, and the cable itself is okay including fuses.
You can test whether the power supply has died by removing the 20(+4) pin connector from the motherboard, and shorting the GREEN wire pin to any BLACK wire pin.
Upon doing this, the PSU should kick into action (any LED if present and fans should come on, as well as power rail outputs), and you would be able to measure 12v from a YELLOW to a BLACK wire, and 5v from a RED to a BLACK wire.
If the PSU doesn't react at all to this, the PSU is dead and will need replacing. If the PSU does react, you can try bridging the pins on your motherboard corresponding to the PWR_SW / POWER_SW. If that doesn't work, it's most likely the motherboard itself has a problem.