If all the other suggestions fail, you can then start swapping a crossover from a known good speaker into the bad one to see if the crossover failed. Chances are there is a loose connection from the main speaker terminal on the back to the crossover circuit board or something went terribly wrong with the crossover itself, odd to have both side of the crossover go dead.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70