In order to power a couple of thermistor controlled cooling fans, I use a pair of USB to 3 pin Molex adapters.
I noticed the other day that one of the fans wasn't working, so I detached it from it's mounting plate and brought it and the adaptor out to check.
Access is a bit... tricky... so I couldn't really test the adaptor against the other fan (and didn't want to risk breaking it if something odd had gone wrong). The fans I use are about £5 each, and it's always worth having spares, so I ordered some replacements, which arrived today.
I plugged one of the new fans into the adaptor and tried to power it on. Nothing. So, I dealt with the access issues in order to plug the new fan into the other adaptor to check the fan worked - it did.
The last remaining check then, was to verify that the issue didn't lie with the USB port the adaptor was plugged into.