Motherboard stopped powering up after Windows update, but after a while, started by itself

Question

I've never seen anything like this before...

I was using my PC as usual, doing so for almost 1.5 years without any issues, then I started updating Windows as usual, which completed successfully, then I restarted my PC so I could verify that all is well and go to sleep.

After rebooting, I enter my PIN to login to Windows, then suddenly Windows begins the process of shutting down for some reason! The screen goes black after, the power button led starts blinking as if it was going to sleep, but the blinking wouldn't stop for at least a minute, at this point I'm guessing Windows is just behaving strangely so I hold the power button and force it to shutdown.

Then, to my shock, I press the power button and absolutely nothing happens, as if the cable between the button and the power pins on the motherboard are disconnected! The motherboards seems to be getting power since the typical white glowing LED is working, and the red power led and green reset led are on, but it doesn't want to actually power up.

I disconnected the PSU, waited for all leds to turn off, plugged it back in, pressed again, nothing. Disconnected PSU again, held down the power button for 30 seconds to flush dirty power out (read it somewhere, tried out of desperation) but still the same result.

I open the case, reconnect the power pins, try pressing the button next to the power led, still nothing. Cleaned out the dust from the PSU desperately hoping that would help, but nope, still dead. No matter what I did, it just doesn't want to power up.

At this point I'm just staring at my open case wondering what just happened, when all of a sudden, out of nowhere, the fans start spinning, the GPU turns of, and the PC boots up! Huh???

I'm glad my PC is working again, but what in the world just happened? I've never seen anything like this before. What could have caused this?

My PC specs are:

• Motherboard: ASUS X99-A II
• PSU: Seasonic Prime 850TD
• CPU: Intel Core i7-6850K 3.6GHz (LGA2011-V3)
• GPU: MSI GeForce GTX 1080
• Storage: Intel 750 PCI-E SSD (1.2TB)
• RAM: 32GB HyperX Savage DDR4 (4x8) HX424C12SB2K4/32
• PCI-E Misc: Archer T9E Dual Band wireless card
• CPU FAN: be quiet! Dark Rock 3

The updates that were installed before this happened:

• September 11, 2018—KB4457128 (OS Build 17134.285)
• Servicing stack update for Windows 10, version 1803: September 11, 2018
• Security update for Adobe Flash Player: September 11, 2018

Could the updates have included some firmware updates that may have triggered this? or was it perhaps my impatient forceful shutdown?

Answer 1

Could the updates have included some firmware updates that may have triggered this?

Microsoft only releases firmware updates for their own devices. Any firmware for any other OEM must be installed manually. There is not a single update for your device, installed through Windows Update, that would cause the behavior you describe.

or was it perhaps my impatient forceful shutdown?

It's possible. However, your issue likely is a hardware issue, not a software issue. Your issue was not caused by the installation of any of those updates.