The working computer is an HP. The broken one is a Dell.
Power supplies tend not to be compatible, especially when one of them is a Dell. There was a time long ago that plugging a "standard" PSU into a Dell mobo would fry the latter. Deliberate incompatibility? Yikes.
That might no longer be true, but I'm not willing to risk it.
As for the fan ... not spinning. Neither the PSU fan nor the chassis fan.
Yes, I have a multimeter. But in my old age I'm inclined not to f*@k with things that I surely would have long ago. So tomorrow I'll look for a repair shop.