CPU: i5 2500k @ 4GHz
GPU: EVGA GTX 970 FTW x2 SLI
RAM: 16GB (4x4gb) DDR3 1600
Motherboard: ASRock Z68 Professional Gen3
PSU: Corsair HX850 - 850W 80 Plus Gold
I built this computer 7 years ago back in 2011. It's been doing fine since but last night I left the room while playing a game and when I came back the PC had shut itself down. I hit the power button and nothing happened. I turned the PSU off with its switch in the back and left it for a couple minutes. When I tried to boot again, the fan lights flashed and the fans started to spin for a very brief moment but ultimately nothing actually happened. Leaving it off for longer, when I tried to boot again it did the same thing but the fans and lights remained on for longer (talking milliseconds here so it's still over before any fans can finish a half rotation) but again nothing actually happened.
Now on to the weird part. I unplugged the bottom GPU and the SLI bridge to check if my PSU couldn't deliver enough power for everything. Nothing happened when trying to boot, but there was a very faint burning smell that almost seemed like it wasn't actually there. I unplugged the top GPU as well (only from the PSU - both cards remained in their slots but unplugged) and suddenly the computer boots. I shut the computer down again and plugged the bottom card back in to see if it's just my top GPU messing up instead of the PSU. The computer booted up again but the bottom GPU's fan was blowing max speed. I shut it down again and plugged the top GPU back in along with the SLI bridge. Strangely, the computer boots up. This is weird as everything is plugged in the way it always has been and earlier it couldn't boot this way, but now it suddenly can. However now both of the GPUs have their fans immediately going max speed and the burning smell is stronger. Fearing that I'm further damaging components, I've left it shut down since and decided to wait for someone more knowledgeable to chime in.
I'm having trouble deciding if it's my PSU, GPU(s), or the motherboard. Maybe it's something else entirely, who knows. I do feel it's worth mentioning that a couple days ago we had a storm that blew a light bulb, tripped the breaker, broke our AC, and fried our modem (actally fried - it sizzled and popped before dying). However this was a full 2 days prior and the computer had been running just fine since the storm until this happened so I don't know if it's related or not. Also I have no extra components laying around that I can swap out for testing so I'll have to find a way to test what I have by itself. This computer is all I have so I'm hoping someone can help me with this. Thanks for any advice you can give! Let me know if there's any more information you need.