Question:  If ALL columns to the right of column D are hidden, how do you "unhide" columns E, F, etc.?

Answer:  In the spreadsheet below, all columns to the right of column D are hidden.

To unhide these columns, click on the column D heading. While holding down the left mouse button, drag your cursor to the right until a bubble window appears displaying "253C". (The text in the bubble window may vary based on your version of Excel.)

Now, over the column D heading, right click and select Unhide from the popup menu.

Now all of the columns to the right of column D should appear.

To learn how to hide a column, click here.