HP 8100 cmt won't turn on, have I accidentally trashed the motherboard?

[A89jop89p5]

HP 8100 cmt won't turn on. One of the two power button led lights for the duration the power button is physically pressed, but it doesn't remain on. The second led by the power button usually doesn't light green as it should, but on a couple of ocassions it has. When it did it stayed lit for maybe 30 seconds before turning off, then lighting permanently red. The fan started up at this time. I have googled and can't find what a permanently red led indicates, the error codes seem to be based on the number of times it blinks red.

Edit - I have just noticed that the second led has just lit up green without my pressing anything lol. This time it has just remained lit green. The fan didn't start up.

I have disconnected all peripherals and pressed the power button for ten seconds.

I reset the cmos.

I disconnected/reconnected any connectors from the power supply to the motherboard.

I tried starting up with various ram chips taken out.

When the power cable is connected a green led between the ram chips on the motherboard lights (showing me that power is getting to the motherboard).

This happened when power to the computer was discontinued while it was on, causing a non-standard shutdown. I then moved the computer before turning it on, but didn't think I damaged it in any way. I updated the bios a few days ago but it has run fine in the intervening time, including (I am pretty sure) turning it off/on on more than one occasion.

Is there anything else I can try?

Replacing the motherboard is not expensive. Is there any way I can find out whether the motherboard is dead (as I assume), or if there is a problem with the power supply, or some other component?

Thanks very much for any help!

[Echo_Lake]

@A89jop89p5
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