Click the PivotTable. This displays the PivotTable Tools tab on the ribbon. To switch between showing and hiding field headers, on the Analyze or Options tab, in the Show group, click Field Headers.
Why can I not see a field in a PivotTable?
Right-click any cell in the pivot table and select Show Field List from the menu. This will make the field list visible again and restore it's normal behavior. The field list will disappear when a cell outside the pivot table is selected, and it will reappear again when a cell inside the pivot table is selected.