Disconnect the PSU from everything. Then on the 20/24-pin ATX connection, short the green wire with any black wire (using a wire or paper clip). The power supply should go on. If it doesn't, then something is wrong with the PSU. If it turns on, then connect only these connections/devices to your motherboard (all power connections mentioned are required if there is the connection/socket on the motherboard): 20/24-pin ATX to motherboard, 4/8-pin to motherboard, insert video card and connect it to power (if required), insert 1 stick of memory, insert/install CPU & heatsink, connect heatsink power to motherboard or power supply. Make sure PSU is on (if there is a switch), then short out with power switch connection with a screwdriver and the PC should turn on. If it does but are still having problems with POST, connect the PC speaker to the motherboard and listen for the beep code and look that up. If it doesn't start but the power supply started when nothing else was connected, then something is wrong. Possibly when the old PSU went out it killed the motherboard. If the CPU, memory, or video card are bad, you would still get a beepcode (i'm pretty sure) and fans/power would still turn on. Try that and let me know.