I've sorted my attribute table by clicking on one of the column headers. Now I'd like to undo the sort and go back to the original table order. Each row has a reference number down the left edge but you don't seem to be able to sort on this reference number.
If using 2.16 you can right-click on the column heading, select 'Sort...' and delete the expression in the window. If you find you cannot manipulate this window, check the box Defined sort order in attribute table in the top left corner.
Try closing and opening the attribute table