Select the entire worksheet by clicking the triangle at the top-left corner between row numbers and column letters, or press Ctrl+A
Right-click any selected column header
Click Unhide
If that does not work, go to the Home tab
Click Format in the Cells group
Hover over Hide & Unhide
Click Unhide Columns
If rows are hidden too, use Home > Format > Hide & Unhide > Unhide Rows
If the sheet is protected, unprotect it first
If columns are hidden due to grouping, expand the outline using the plus buttons or Data > Outline > Ungroup
If columns are hidden by a filter, clear the filter
If columns are hidden very far apart, select the whole sheet and use Home > Format > Hide & Unhide > Unhide Columns