Open a new workbook. Paste the following code into a VBA module in it:

Public Sub UnhideAllSheets()
'Unhide all worksheets  in the active workbook.
Dim x As Integer
On Error GoTo UASerr
For x = 1 To Worksheets.Count
Sheets(x).Visible = True
Next x
Exit Sub
UASerr:
MsgBox "Error", , "UnhideAllSheets"
End Sub

Open the workbook  with the hidden  sheet. From the Tools menu, select Macro >>
Macros >> UnhideAllSheets >> Run.