This is a long time problem that recently became a real annoyance every single day.
When I press the power on button of my computer in the morning or after a few hours turned off, the power LED brights and the fans start spinning, but usually in less than a second everything goes off and then restarts entering in a neverending loop that prevents the computer from booting up.
In the past I've managed to workaround this by disconnecting the power supply, pressing the power on button a few times and then trying again, or just waiting a few minutes. Later on the problem went worse and I imagined it was related to a faulty PS, so I've replaced it with a different model of another brand.
To my surprise the problem is not gone and looks worse every day. In my tests to identify the cause of the problem I've unplugged different external devices (USB HDD, audio jacks, bluetooth adapter, wifi antennas...) with no luck. I even thought it was related to one side panel of the case that could be producing a short, but taking away this part didn't solve the problem either.
The interesting thing is that if I boot right after turning off, it just goes fine. Is after a long period of time when the problem arises. Maybe the fact that the computer is plugged to a power strip that every night I turn off could indicate something. I have to see what happens in the morning if I keep the power strip on by night.
Motherboard model: Asrock Z68 Pro3
Update 1: this night I've left the power strip on and booting was normal
Update 2: another night plugged on but the morning after I had to do the workaround once
Update 3: the workaround is no longer helpful and it doesn't matter if I keep the power strip turned on. I opened the case and capacitors look fine. I started to unplugg everything from the motherboard (buttons included) and after removing the PC speaker the computer booted. Then I plugged again the rest of the components and with one of the fans that has an extension cable the problem began. So I plugged the fan again without that cable and I managed to boot again.
All this is very strange to me. I doubt these two components can be causing the problem but who knows... I have to wait until the next time I start the computer again to discover it.