How to show hidden columns in QGIS 3.6.1 (?)

Right click on a column, or the grey area where the columns were.

Choose "Organize Columns..."

Retick the hidden columns.

If you have hidden all of the columns, you have run into a bug: https://issues.qgis.org/issues/15471

How would you fix this?

Option 1:

Re-add in the data and copy over the symbology from the old layer. Just copy the symbology.

Option 2:

Edit the .qgs project file. You need to extract the .qgz project into its component parts a .qgs and a .qgd. Open the .qgs file in a text editor. And find the:

<attributetableconfig 

That corresponds to the correct layer.

Then edit the lines like:

          <column width="-1" type="field" name="powsc_csoed_label" hidden="1"/>

To:

          <column width="-1" type="field" name="powsc_csoed_label" hidden="0"/>

Try toggle Editing, add a new column. Now you can right click on the new column to get to organize columns menu. Once you've got your columns back, delete the new added column.