File Open

When in the "Open" dialog box, click on the "View" menu (little button in the upper right corner)  and make sure you select "Details". This will display column headers on which you can click to sort by that item, and click again to reverse the sort order. You should see a little triangle on the column header which indicates what's the current sort filter. So my guess is that this was changed at one point without you realizing it. This is standard functionality in Windows btw, very practical..., especially when you sort by more than one column with the use of the "Shift key" and click. But that trick does not work in all sortable windows though.