There are 3 wires labelled positive. Presumably you are referring to the white one?
If you take the controller off the wall, remove the white wire, and peek into the hole where the wire is supposed to clamp down (may need a flashlight), you may be able to see how the wire is supposed to be clamped down. My guess (and it is only that) is that you will see a small steel rectangular plate, more or less 1/4 inch square. (It may be brass-colored.) It will have a hole drilled in the center of it. The walls of this hole may be threaded, so that when the screw turns as it passes through it, the plate moves up and down. The idea is that you back the screw out a bit, creating a small gap above the little plate. Then slip the end of the wire into the gap you just created above the plate, then tighten the screw, pulling the plate up toward the screw head, and trapping the wire If the screw was backed out too far, it may have come completely out of the little plate. Getting it threaded back in will be a challenge. If you are lucky, tweezers and a steady hand will help.
Another similar possibility is that the hole in the little plate is big enough that the screw slips right through it. The screw is then intended to thread into a fixed contact beyond the little plate. Again, the little plate is what is supposed to clamp the wire.
Remember that both of those ideas are just possibilities - I have no specific knowledge. However, I have seen connections that work this way in other devices.
If I am anywhere near correct, the pieces need to be reassembled in the correct order, with the screw properly re-threaded. I don't have any good ideas how to do this through a small access port. I think you may be faced with opening the case of the device, whether it is supposed to open or not If there are rubber feet on the bottom of the box, see if you can pop them off. Often there are screws hidden beneath the rubber feet.
Good luck
Bill
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