Select the columns around the hidden area (e.g., select a column to the left and the column to the right of where columns are missing).
Go to the Home tab.
In the Cells group, click Format.
Click Hide & Unhide.
Click Unhide Columns.
If columns are hidden due to a filter: go to the Data tab, click Clear (or remove the filter) to show all rows, then recheck for hidden columns.
If columns are hidden via column width: select the full sheet by clicking the Select All button (top-left corner), then go to Home > Format > Column Width, enter a width, and click OK.