I bought a new PSU for mine today, and after wiring and firing it up, well, it didn't. This time there was a light on the motherboard, so I knew there was power there, but it was otherwise as dead as a Norwegian Blue.
After swearing quite a lot, and dreaming up ways of fire-bombing PC World in the morning, because the 12v power connector only had six terminals instead of the eight that the last PSU had, I thought I might as well just try the other power connector, which only had four terminals, and bugger me, it worked, and I'm all up and running again.