So this morning my computer wouldn't turn on. Pressing the power button has no affect. When I turn off the switch at the back and push the power button after turning it back on it seems for about half a second that everything works: fans appear to start moving and the PSU light turns on. This happens only for a brief moment (less than a second) and than everything turns back off. I can repeat this by turning Off and On the switch at the back of the case or by unplugging the power cable out and plugging in again. There is a small led on the motherboard which is always on when power is connected (doesn't matter if the computer is on or off) and it is still on now.
I am trying to diagnose what is the cause of the problem PSU or motherboard, any ideas?