Following example uses InputBox method to validate user entry to unhide sheets: Important thing here is to use wrap InputBox variable inside StrPtr so it could be compared to '0' when user chose to click 'x' icon on the InputBox.
Sub unhidesheet()
Dim ws As Worksheet
Dim pw As String
pw = InputBox("Enter Password to Unhide Sheets:", "Unhide Data Sheets")
If StrPtr(pw) = 0 Then
Exit Sub
ElseIf pw = NullString Then
Exit Sub
ElseIf pw = 123456 Then
For Each ws In ThisWorkbook.Worksheets
ws.Visible = xlSheetVisible
Next
End If
End Sub