I had some time on my birthday to devote to repairing the Mac Mini with the broken sensor cable. The replacement part had arrived, so I found somewhere with bright light, got my magnifying glass ready, and got to work. I put the sensor in place, after a struggle. Then I reattached the hard disk and the Superdrive, and then attached the body to the base. Then I could attach the end of the temperature sensor to the socket on the base. It didn't want to go in. After a lot of fiddling with the magnifying glass, I found that one pin in the socket was bent. I straightened it. Then the plug went smoothly into the socket. I plugged the power in, turned it on to see if the fan would work properly, and ... nothing. It wouldn't turn on. Maybe it needs the lid on before it will turn on? I put the lid on, still nothing. I spent another hour trying to see if I had done anything wrong, and the end result was still ... nothing.
Then I realised that I had missed a very important step last week. Before I ordered the replacement temperature sensor, I had failed to turn the Mac Mini on to see if it still worked. It had been turned off for about a year. It may have just died in that time, and I didn't check, I just ordered the new sensor. That was a mistake. Now I don't know if it was something I did while getting the sensor in, or if it had died before. Now I will never know.
So what do I do with a Mac Mini that doesn't turn on? I'll wait a little while and have another look at it. If I still can't make it work, I'll strip it down for parts and sell the parts online. Get some money back on it.