Question No longer starts up— is it the PSU or the Mobo?

[berv]

I have an MSI Z170A M5 with a 750W power supply, both of which have done me well for the five years up until this morning. At about the moment during idle operation that my computer might have put the monitor to sleep, it instead turned the system completely off and now won't turn on again.

I have pulled out all the peripherals, down to one stick of memory (I've swapped each I have in turn), and reset the CMOS. The only sign of life comes if I hold the power button for 7 seconds, after which a blue LED offers a blink code that is only referenced in the manual as 'Debug', and MSI support could not explain further.

I've done the paperclip test and the PSU fan spins up fine.

As a desperate attempt to learn more, I plugged another PC's 450W power supply in (but only the 24 and 8-pin connectors). Same result (blue blinking, no start).

Is this enough information to conclude that it's definitely an issue with the motherboard? If so, is it possible to pin it down rather than replace the whole thing?

EDIT- additional details:
MB: MSI Z170A Gaming M5 ATX LGA1151
PSU: Thermaltake Smart 750W ATX 12V V2.3 80 Plus Bronze Power Supply 120mm Fan
CPU: Intel Core i5 6600 3.3GHz 6MB 4-Core S1151
GPU: Zotac Geforce GTX 1080 8GB 256Bit GDDR5X
RAM: 2x8GB Kingston HyperX Fury Black 2133MHZ DDR4
O/S: Windows 10 Home
HD: 1TB 7200RPM 64MB 3.5" SATA3

 

[Ralston18]

Update your post to include full system hardware specs and OS information.

5 years old is pushing "age" of a PSU.

Especially if the PSU has been heavily used for gaming, video editing or even mining.

Make and model PSUs ( 450 watt and 750 watt)?

And try the 450 watt PSU but be sure to make all connections per the motherboard and PSU user manuals.

However, it is likely that neither PSU may not be capable of supporting your computer's power demands. Disconnect all extra peripherals. Boot with integrated graphics (if available) versus a GPU.

Objective is to determine the motherboard is functioning.

Do you have a multi-meter and know how to use it? Or know someone who does?

PSU's can be tested beyond the "paper clip" test.

https://www.lifewire.com/how-to-manually-test-a-power-supply-with-a-multimeter-2626158

Not a full test because the PSU is not under load. However, any voltages out of tolerance are a good indication that the PSU is likely at its' designed EOL (End of Life).

After thought: has the CMOS battery ever been replaced?

 

[punkncat]

Assuming that I was in the correct manual, I don't see anywhere that states your motherboard blinks blue. It specifically mentions red and green. ~page 42

I note that you should have an LED display that is capable of showing numbers. In the manual it references debug as a chart where those numbers will relate to a specific trouble as detected by POST.

I also note a wonky overclock "dial" installed on the board?

 

[berv]

@ralston Additional details in OP.
I found a multimeter and tested the 24 and 8 pin connectors; all was within tolerance. I no longer have access to the 450W PSU and am not sure of its details. I replaced the CMOS battery at your prompt but that does not seem to be the culprit. All present tests have been with integrated graphics.

@punkncat I'm losing my mind as to where I read that bit about the LED, but the one that blinks is undeniably blue. Sits in between my PCIe slots, near the CMOS battery. Unfortunately the numerical display doesn't light up at all in my attempts. As for overclocking, there's a hardware SLOW switch on the board that I've cycled to no avail