Select the columns on both sides of the hidden columns
Right-click the selected column headers
Click Unhide
Select the entire worksheet by clicking the Select All button or pressing Ctrl+A
Right-click any column header
Click Unhide
Go to the Home tab
Click Format in the Cells group
Point to Hide & Unhide
Click Unhide Columns
Use the Name Box to select a range that includes the hidden columns
Right-click the selected range
Click Unhide
Press Ctrl+Shift+0 to unhide selected columns if enabled on your system