If you import or export FBX files and the FBX Importer or Exporter window does not appear, it could be because the Maya FBX plug-in cannot find MEL scripts that are included by the FBX installer. The scripts are automatically installed to the following path:

<Maya version>/scripts/FBX

If they are deleted, the FBX user interface no longer appears. You can restore these files which will bring back the interface if you reinstall the plug-in.

If you are using the Maya Default UI setting, the FBX options to the right of the file browser area may be collapsed. Drag the splitter bar at the right side to the center of the dialog box to expand the FBX options.