Motherboard won't turn on, green LEDs come on, but no power.

Although you may have already done so...

Check the 24-Pin ATX Power Connector on the Motherboard and the 8-Pin CPU Power Connector. Make sure both are firmly and completely clipped in to their sockets. Even a very slightly loose cable can result in non-functionality. Ensure these cables 'click' into their sockets.

You could try prying the connector of its socket using a screwdriver with a very thin head or a set of pliers. Be very careful and gentle however as you don't want to bend / break any pins. You can also expect some cosmetic damage to the plastic frame of the socket.

Make sure you're holding the clip in the open position. Don't pull on the cables as you could damage the connector. Grip the plastic frame of the connector head.

Moved paragraph 3 to the top, to cut a long story short - my PSU is affected by temperature, it musta been about 4 degrees C in the room I had my PC in, heating it up with a fan heater for 5min would've saved me 2.5 hours of troubleshooting. I tried it as a last resort out of desperation, and because I knew from studying physics that heat affects electromagnetic induction and static electricity, so it wasn't totally mad, but I still found it mad that it worked first time like there never was any power/PSU issue... if anyone says they have an LED blink followed by nothing, ask them the temperature of the room it's in, don't just tell them their PSU is borked, even though it might be, as PSUs do break on occasion. My main point is that temperature in cold places/at cold times of the year should be part of the troubleshooting manual for static/EM induction/twitchy PSUs I hope someone spreads the word in troubleshooting circles for the exact problem I had and saves other people thousands of man hours, even if it is troubleshooting step 11 after the other 10 steps.

I just spent 2 and a half hours trying to figure out a quick flash from MB and DVD drive LEDs, followed by silence and no second flash even if I tried tapping the power button again, my setup doesn't even have a PSU LED. This is probably totally irrelevant to your situation and I realise failed motherboards do cause this and have seen such written on other threads regarding new PC builds, but I wanted to share the solution in an active thread, with hopes people will copy/paste it or just remember it for other people who might have the exact same problem I had.

I tried all the normal troubleshooting, re-housing pieces, checking for shorts/static, CMOS etc. and was stumped as to what the problem was, suspecting PSU failure, this isn't the first time it's happened with the same PSU and it has come back to life eventually, I thought in previous successes that fiddling with the power switch lead on the MB solved it, but despite trying that many times, it failed to resolve the issue this time.

That's strange. Generally speaking lower temperatures are a good thing for all electronic components except batteries. Lower temperatures reduce resistance on electronic circuitry resulting in more efficient performance. Higher temperatures can cause cumulative heat burnout increasing resistance causing increased power input which exponentially increases temperature and thus resistance until the component melts or fries.

Still, the evidence is what it is. I'm not arguing with you but my knowledge of PSUs isn't sufficient to explain what part of that component would be hindered by a low(ish) temperature.

Yea, I realise CPUs, GPUs and other small electronic chips require cooling and work better at lower temperatures, but larger electronic devices have different operational temperature ranges dependent on a few factors. Two of the key factors I believe in this instance are size and material it's made of, the PSU contains a transformer (naturally) and a transformer is a set of at least 2 coils of wire, sometimes made from different metals, which exchange and convert voltages and currents up or down. As you pointed out, heat changes the resistance and therefore the behaviour of the metals in question, meaning they output different voltages/currents than under normal conditions, leading to my PC not booting up.

I suspect larger transformers like those you find outside power plants have to be carefully designed to try to cope with min/max temperatures in that country. Most PSU's should be designed to cope with those min/max temperatures too, so maybe mine's a cheapskate one, with an unstable metal used in the coils or other parts of the component. Copper is normally used in the transformer coils, maybe mine doesn't use it? Actually most components inside should be made from copper, which is quite stable over a wide temperature range, both high and low, so something else must be the material used in one of the components of my PSU.

The second possibility I can think of is some form of static, in the PSU, on the MB or some other component that'd stop a PC booting, I think this is less likely to be the case, but still - a possibility, and heat also alters the charge that can be held on the surface of any material, plastic, metal or other. I just hope someone picks up this point and adds it to a standard troubleshooting guide, and saves a lot of people hours, it woulda saved me hours, had I known - so wanted to tell the forums about it.