PC won't boot sometimes

[adamn]

This problem first occured couple of weeks ago, when I turned it off and then tried to turn it on it wouldn't. After pressing the power on button the fans start to work but nothing else happens, no signal on monitor, no signal to the LEDs in my mouse (Steelseries Sensei) and can't enter BIOS menu. After wating for a couple of hours I try to turn it on again and it does turn on, but when I try turning it off and then on again it doesn't. When it turns on properly 2 red lights on the motherboard flash. I reinstalled Windows, and it worked for a day and now I have the same problem. I even tried CMOS resetting.

My specs:
Intel core i5 4670k
Asus GTX 760 OC
Coolermaster Hyper 212 EVO
Asus z87-a
Fractal Design Arc Midi R2
Kingston hyperx 8GB DDR3 RAM
Kingston 120GB SSD
WD 500GB
Corsair CX600M
Windows 10

 

[WyomingKnott]

My first guess would be a dead power supply. Question: Did this machine work before? For how long?
If it were my system, I would
1) Use my power supply tester on the power supply
2) Unplug and replug all cables between PS and motherboard, including the power switch
3) Test with a spare power supply (you have a box of those, right?)