Check AC power,make sure the socket it is plugged into works by trying another appliance. Check the fuse in the power lead plug. There is a small green light on the motherboard, mostly you can see it through the side casing but you may need to remove the cover. It should be on when the AC power is applied to the PC. If it's off it will probably indicate a bad PSU. If it's on you may have a defective power on switch or bad PSU but you'll need a multimeter and some knowlege to check further.