Apple Mail: no search results after Sierra update
I would start with the procedure described in Rebuild the Spotlight index on your Mac:


  
  Choose Apple  menu > System Preferences, then click Spotlight.
  Click the Privacy tab.
  Drag the folder or disk that you want to index again to the list of locations that Spotlight is prevented from searching. Click OK to
  confirm.
  Select the folder or disk you just added, then click the Remove (–) button to remove it from the list.
  Quit System Preferences. Spotlight will reindex the contents of the folder or disk.