The thermal protection feature of the CPU kicked in, luckily!
At first I didn’t know what had happened, as the machine got to a point where it wouldn’t POST, and wasn’t displaying a video output.
When I opened the machine up, I noticed that the fluid wasn’t flowing in the cooling loop, and there were bubbles on either side of the cooling block, indicating that the fluid had started to boil!
I’m still going to open the pump to see what happened internally. I suspect that the ethylene glycol clogged up the bush that the impeller sits on! I’ll report back on this!