Numbers

Perhaps you hid it by mistake? If so you could try clicking in the table and from the menu choosing Table > Unhide All Columns.

If it isn't hidden then maybe you deleted it by accident? You could try going to File > Revert to > Browse All Versions ... and go back in time until you find a version of the document that still has the deleted column.

SG

While it can be seen in SGIII's screenshot, it should be mentioned that if you have a hidden column its column letter will be missing. If you see columns A, B, D but not C then C is hidden. If you delete a column, though, all the columns to the right of it will get renamed. D will become C, E will become D, etc.