AC in the attic affects downstairs but not upstairs
Generally, there are not 2 fans. Instead, there are electrically controlled dampers that open and close based on signals from a control unit. The damper for your upstairs vents is probably jammed.

The control unit for the dampers is usually mounted on the wall near the air handler, or directly on the air handler. There should be 4 to 6 wires coming out of the controller: one for each thermostat, one for each damper, and possibly one or two more going to a power supply or to the air handler. They're probably labeled if you open the cover. Follow the cables to find the dampers.

The dampers that I have in my house have a cover that you can pop off to get at the damper and move it by hand.