If you've ever hidden a column in Excel and then found that you can't unhide it, even when you right-click and choose "Unhide", you're not alone. It's a common problem, and one that has a few different causes.
The first thing to check is whether the column is actually hidden or just very narrow. To do this, select a cell in a row to the right of the hidden column. If you can see the cell contents, then the column is just very narrow and you can unhide it by dragging the column boundary to the left.
If the column is truly hidden, there are a few possible causes: