No video, PSU runs fans, no beeps from case speaker

I left the desktop I built unplugged for a week while I was on vacation, but now I find that it's unable to boot in any normal way. The PSU seems to be able to run, at least with a tiny load (paper clip test with a few fans and LEDs hooked up), but whenever the motherboard is involved, the whole system will shut down after a minute or so and then turn back on about 1-5 seconds later.

While it is on, the "Phase LEDs" on my motherboard stay fully lit the entire time. These are supposed to indicate CPU load. I don't know if they would normally be fully lit or not when idle in BIOS. The CPU fan spins at a noticeably increasing rpm until the system shuts down. When the shutdown occurs, the Phase LEDs flicker on and off for a few seconds before turning off. Immediately afterwards, about a second or so later, the system turns back on and the process repeats. At that point I shut it down by holding down the case "power" button, which works as it normally would.

The motherboard does not send video through the GPU or through its own integrated video output, so I cannot see how far it gets into the boot cycle before shutting down (no speaker). Does not seem to be caused by any SATA devices (all are disconnected) or the RAM (tried both sticks in slot 1, one at a time) or the GPU (removed).

The CPU fan turns on and starts spinning faster and faster until the shutdown occurs.

Unfortunately, I can't test any of these parts individually because I only have one of each.

All parts were new to this build. The system has been heavily used for the past 8 months and has never crashed.
PSU fan sometimes does not spin, but Corsair support told me that this is normal for this model.

I did not make any major changes to the system immediately prior to this, aside from unplugging some USB devices and using a different ethernet cord for a few minutes to transfer some files between computers. I did accidentally leave the PSU set to "1" while I unplugged the surge protector from the wall a few seconds after completing a normal Windows shutdown. Could that have fried something?

i5 2500k
Motherboard: Gigabyte GA-Z68XP-UD3P
PSU: Corsair CMPSU-650TX 650W
RAM: G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
(RAM was proper voltage for this - 1.5v)
CPU cooler: CM Hyper 212+ & arctic silver 5 thermal paste

Things I've tried:
- Removed all non-essential devices (GPU, SATA)
- Booted with a single stick of RAM in slot 1; tried both sticks
- Paper clip test on PSU (test passed, running one 120mm fan, one 200mm fan, and one LED simultaneously)
- Reset CMOS by removing battery
- Breadboarded the system
- Motherboard's internal video output is an HDMI port. My system does not output through my HDMI->DVI adapter into my monitor, but I am using this adapter and monitor on my laptop with no issues.
- Removed the internal 2tb HDD - still works fine and have encountered no file corruption or unusual behaviour

- Is it safe to test this system (just to BIOS, not into Windows) with a known-working 500w PSU that has a 4-pin CPU plug? My motherboard has 8 and I have been using 8.
- See below for more.
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More about video runs fans beeps case speaker
  1. I installed a case speaker. When ram is in, it makes NO beeps. When ram is removed, it makes a continuous beep. This indicates either "graphics card not inserted" (long) or "power problem" (short). I can't tell if the beeps are short or long since I've never heard them. Probably short though?

    Also, I think a case fan has now been damaged as it's no longer spinning when connected.

    Video of the problem:
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