Long story short: desktop computer was acting up, so I held down the power button to temporarily cut power to it -- but ever since, it hasn't responded to anything I've done.
The fuller story: I use a custom-built desktop PC which hasn't given me any trouble until today. I had put it to sleep on leaving the house, and when I came home, I tried to wake it up (by clicking the mouse button, which normally works). The computer seemed to respond -- i.e. the lights on the tower changed and I heard the fan kick in -- but nothing happened on the screen. Thinking it was probably a monitor issue, I double-checked the monitor's connections, but that all seemed to be working (i.e. the connections were all in place; the monitor was receiving power and showed its start-up screen when I turned it off and on).
Thinking that a simple restart might solve whatever was going on, I pressed the restart button on my tower, but that didn't seem to do anything. Then -- probably hastily -- I held down the power button, cutting power to the PC. That was the last time I've seen any lights on the tower, or gotten any response from the computer whatsoever. Ever since holding the power button down for a few seconds, it's just had all the symptoms of a PC with no access to power: the buttons don't do anything, there are no lights, nothing.
I'm pretty certain that it *is* getting power. I've put it in a couple of different power outlets that are currently lighting lamps, and gotten the same (null) result. I've double-checked the connection between the power outlet and the power supply, and I don't see anything wrong. I'm not sure what was going on with the computer at the beginning (i.e. why it wasn't waking from sleep like normal), but now I'm just trying to deal with this more fundamental problem, which is -- how can I make the power button turn it on again?
Any help would be massively appreciated. I really need this computer back ASAP for my study. I'm running Windows 10, and here are some hardware specs, on the off-chance they might be relevant:
POWER SUPPLY: Corsair SF600
MOTHERBOARD: Gigabye G1 Gaming GA-Z170N (Mini ITX)
CPU: i5 6600k
GPU: Sapphire AMD r9 390x 8gb
RAM: 16gb DDR4 Ripjaws
SSD: Crucial MX300 1tb SSD
HDD: WD 1tb
CASE: Bitfenix Prodigy