When this happened to my right mirror, the nut came all the way off the bolt, and I couldn't get it back on through a slit across the bottom of the accordion. I cut the entire back off the accordion, and it's ugly now. Clearance on the faring also hindered my movement.
I wound up replacing the entire bolt and nut with an allen bolt. This let's me use the hex wrench coming up from the bottom, easily clearing the faring, and a 10mm bolt on top, easily accessible with a socket wrench. I keep it nice and tight.