You could use a macro to unhide all the sheets.

Option Explicit
Sub testme()
   Dim sht As Object
   For Each sht In ActiveWorkbook.Sheets
       sht.Visible = xlSheetVisible
   Next sht
End Sub

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm

Running this macro should work..

Sub UnhideAllHiddenSheets()
 Dim WS As Worksheet
 For Each WS In Worksheets
   If WS.Visible <> xlSheetVisible Then WS.Visible = xlSheetVisible
 Next
End Sub

but if you have some sheets that are Very Hidden and you want to keep them
Very Hidden, run this macro instead...

Sub UnhideAllHiddenSheets()
 Dim WS As Worksheet
 For Each WS In Worksheets
   If WS.Visible = xlSheetHidden Then WS.Visible = xlSheetVisible
 Next
End Sub

Rick