Have you also had the HDD/floppy unplugged? If the IDE is in wrong it will seem like everything is dead. If the floppy is in wrong, the light will stay lit. Goofy, but if a LED is jumping reset, it can also seem dead.

What changed when it stopped working (if it ever was)?

Hi Major Jackyl, it never worked. This is my first foray into a late 90s machine, and after I put all of the parts together, the problem I described in the main post started happening. I’ll try with the floppy and IDE devices (DVD drive, HDD) unplugged and get back to you

Hi. If this is of any help - this looks exactly like my 440BX (a cheap Lucky Star 6ABX2C) passed away this winter. No POST. The floppy and the HDD would blink, and then nothing happens. If I waited long enough (I mean long indeed - like 30-40 minutes), the system did start. However, it took longer and longer each time, so this was finally unbearable. I tend to blame the overheated BX chip, which in my case had no cooling and must have worn off after all these years.

https://theretroweb.com/motherboards/s/compaq-garry

Remove everything. Attach speaker. Listen.

Hey everyone, I disconnected the FDD and HDD from the motherboard and it boots up!!

I get “162-System Options Not Set”, “304-Keyboard or System Unit Error” and also “163-Time & Date Not Set”, and it also tells me “CMOS checksum is invalid, default values loaded”, however all of them (apart from the keyboard one) are probably just from me resetting the CMOS.

I don’t have a PS/2 keyboard so I’m going to try a few USB keyboards with a PS/2 adapter.

Unless you've got a really old usb kb, or an active adapter, it's not likely to work.

Ps2 and usb aren't compatible.

But there was a period where simple pin adapters existed - because the keyboard supported both protocols.

It's likely you have a usb only kb and a simple passive pin adapter and it won't work.

Yeah I’ve come to realise this. Bought a PS/2 KB for £5 though so it’s all good