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First need to select all cells:
Press Ctrl + A Then use any of the following:
Method one:
Press Ctrl + Shift + 0
Method two:
Press "ALT" + H + O + U + L // each key at a time
Method three:
Go to "Home" → "Format" → "Hide & Unhide" → "Unhide Column"
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'Step1: select the area where you want to unhide columns.
'Step2:
Selection.EntireColumn.Hidden = False