I'd plug the USB cable in with no printer at the other end and see what happens. Then I'd replace the USB cable and see what happens. Essentiall try and see if the cable is faulty.
PSU's are funny things. They can suddenly pump out enough power at the right voltage to turn a fan, but not enough to make the electronics work. They have more than one output. The worst thing that can happen is the fan not get power to turn and you fry your CPU.
Usually when a PC suddenly fails to boot something inside has become not connected properly. Components need to be pushed into place, then replaced one by one to see what is disconnected. Usually that is.