Need some help troubleshooting new computer's failure to boot.

[Avarice]

So, I built a new computer (my first time building one) using mostly new parts, but a couple old parts as well.

Operating System: Windows 8 Pro
Case: Antec Three Hundred Black Steel ATX Mid Tower
Processor: Intel Core i5-3570 Ivy Bridge 3.4GHz || LGA 1155 77W Quad-Core
Motherboard: ASUS P8Z77-V LK LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 ATX (UEFI BIOS)
Memory: 120GB SSD - SATA III (in addition to 5.5TB in External HDDs)
Video Card (reused): SAPPHIRE (AMD) Radeon HD 6870
RAM (reused): 16GB DDR3 (Ripjaws 4x4GB)
Heatsink: COOLER MASTER Hyper 212
Power Supply (reused): Corsair GS700

When I finished putting everything together, it booted up fine, I installed the OS without any problems, and it ran perfectly for approximately 12 hours.

Around the 12th hour, the computer just randomly shut off for no apparent reason. When I turned it back on, the power turned on, everything lit up, and it stayed on for about two seconds, then turned back off. After another second or two, it turned itself back on on its own, and this time it stayed on, but there was no display on the monitor. I opened up the side of the case and looked inside; the fans attached to the power supply, the GPU, and the heatsink were all spinning, but the two built-in fans attached to the tower were not.

Upon closer inspection, none of the wires had moved, but I moved one of the two fans' cords to a different chassis_fan slot on the motherboard, and that fan started working again while the other remained off. I also tried removing the GPU, and using the built-in video chip on the motherboard, but there was still no display. I felt around the CPU and Heatsink and it didn't really seem to be hot at all.

If anyone has any ideas, I'd greatly appreciate it. Thanks.

Also, if this didn't warrant a new thread and deserved to go into the RQT thread instead, I apologize.

[Avarice]

Just an update; I tried booting with only one stick of RAM plugged in versus four, and this time the previously unworking fans now work, and the computer didn't do its on-off-on routine, but there's still no video.

[Niya]

Hm, sounds like a really strange problem... First guess on the random shut downs would have been heat issues. But that shouldn't cause loss of video output...
The case fans are both directly attached to the mainboard? I somehow get the feeling you have a faulty board... That could explain the fan control going nuts and probably also the shut downs. Fans spinning up sometimes just means that the board gets power, not that the actual boot has started.
Do you get the POST beep on boot, do you see (LED) or hear HDD access? Are there any signs for how far the system actually boots? You could also just for fun provoke an error beep code by booting without any RAM just to see if POST works properly.

[GoggleHead]

My money's on it being a borked motherboard.

Did you actually remove the GPU entirely, or did you just plug your monitor into the onboard video? Some motherboards will just lock the onboard video out if there's a GPU plugged in.

[xopher]

Need all of your specs. Primarily CPU(brand/model), GPU(brand/model), and PSU(brand/model/how long you've had it) and exactly how many HDD and Optical drives you're using. Are you doing any overclocking?

I'm thinking it's a PSU issue.

[Crowort]

Pretty much what Xopher said. First thing I thought was PSU.