Strange question here. Never had this problem before. I can't get the order of the metadata order to reverse in the Metadata Column. I would like it to be A-Z in every column. Is this just a simple setting that I'm missing? It's driving me crazy.
Not all fields allow you to sort Ascending or Descending. It's possible that you had changed the column to Descending, then changed the field to one that doesn't have that sort order menu option. If you change the column to Date, you can change the order again. Or restarting LR may clear it.