, -tech- follow instructions before you computer.jpg
1. Are you just pressing the power button on the front, or do you have a switch on the PSU as well (not all do). Unplug your computer, then plug it back in. It's sometimes possible for (hybrid)hibernation or sleep modes to fuck up and not allow the computer to wake again for a variety of causes. Disconnecting the power physically will typically fix this. Also, hold the power button for 15 seconds, then press it again.
2. Test the outlet and any surge protectors/extension cords that you are using by plugging in a lamp or something.
3. Re-seat the various power connections. They could have developed a poor connection somewhere due to thermal cycling, and were barely making contact when the computer was warm, but have contracted now that the computer is cool.
At that point I would use my spare power supply I have leftover to check if that works. If it didn't work, they it could be a motherboard problem. Often PSU and motherboard failures are the result of a capacitor popping and making "magic smoke" which is typically very noticeable, but not always. You should look over the components for anything that seems damaged/warped/out of place. Unfortunately there are plenty of components that can also die silently for a variety of reasons.