Lightroom 4: “Sort” option/dropdown box missing from Library view

  This one isn’t related to computer parts for a change but is something that happens to me regularly – I’ll import a handful of photos and want to sort them by capture time, but the “sort” box is missing from the Library view in Lightroom 4.0. You can bring it back by pressing T.   I don’t know how it is I keep on accidentally hitting T but it seems I must do so regularly judging by how often this happens…