LO Version: 7.5.0.3 (X86_64) / LibreOffice Community
Build ID: c21113d003cd3efa8c53188764377a8272d9d6de
CPU threads: 4; OS: Windows 10.0 Build 22621; UI render: Skia/Raster; VCL: win
Locale: en-CA (en_CA); UI: en-US
Calc: CL threaded
I hide a column. I click Format–>Column–>Show on the column and nothing happens.
As I said, that does nothing.
Ok, now I get it. To unhide one column one has to select more than one additional columns to show the one initially hidden.
Yes preferably the columns to the right and left of the hidden column 
Strictly speaking, just one cell in the hidden column is enough.
You can enter the cell address in the name box (Ctrl+Shift+T), then choose menu Format - Columns - Show.
This is useful when more than one column are hidden, and you must show only one.
Great tip! Thanks!